build: Add pango to pkg-config

libgimp/gimp.h includes pango/pango.h but pango was only listed in Requires.private field of gimp-3.0.pc. Similarly, it includes gexiv2/gexiv2.h through libgimpbase/gimpmetadata.h.

This would cause projects linking against gimp-3.0.pc to fail to find the headers when using alternative interpretation of pkg-config specification that only considers private dependencies for include path during static builds, such as the case e.g. on Nix.

https://gitlab.freedesktop.org/pkg-config/pkg-config/-/issues/28

[87/98] Building CXX object CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o
FAILED: CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o 
/nix/store/4apajimszc47rxwcpvc3g3rj2icinl71-gcc-wrapper-13.3.0/bin/g++ -DGIMP_DISABLE_DEPRECATED -DGMIC_HOST=gimp -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_NO_DEBUG_OUTPUT -DQT_WIDGETS_LIB -D_IS_UNIX_ -Dcimg_appname=>
In file included from /build/source/src/Host/Gimp/host_gimp.cpp:25:
/nix/store/4cqvgngq4mpgdwzjvn7pk2d5mmg3kqq3-gimp-3.0.0-RC2-dev/include/gimp-3.0/libgimp/gimp.h:28:10: fatal error: pango/pango.h: No such file or directory
   28 | #include <pango/pango.h>
      |          ^~~~~~~~~~~~~~~
compilation terminated.

FAILED: CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o 
/nix/store/4apajimszc47rxwcpvc3g3rj2icinl71-gcc-wrapper-13.3.0/bin/g++ -DGIMP_DISABLE_DEPRECATED -DGMIC_HOST=gimp -DQT_CORE_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_NO_DEBUG_OUTPUT -DQT_WIDGETS_LIB -D_IS_UNIX_ -Dcimg_appname=\"gmic\" -Dcimg_display=1 -Dcimg_use_abort -Dcimg_use_cpp11=1 -Dcimg_use_curl -Dcimg_use_fftw3 -Dcimg_use_openmp -Dcimg_use_png -Dcimg_use_vt100 -Dcimg_use_zlib -Dgmic_community -Dgmic_core -Dgmic_gui -Dgmic_is_parallel -I/build/gmic-qt/build -I/build/gmic-qt -I/build/gmic-qt/build/gmic_gimp_qt_autogen/include -I/build/gmic-qt/src -I/build/gmic-qt/../src -isystem /nix/store/05w0r4yjf3adhhxgajfjki8kcrh97xjd-harfbuzz-10.0.1-dev/include/harfbuzz -isystem /nix/store/g8psrwxldnl8hnksf0rind77ja4pjwxc-glib-2.82.1-dev/include/gio-unix-2.0 -isystem /nix/store/g8psrwxldnl8hnksf0rind77ja4pjwxc-glib-2.82.1-dev/include/glib-2.0 -isystem /nix/store/26hcp8h792wl0h52c5r94qakhvk6q717-glib-2.82.1/lib/glib-2.0/include -isystem /nix/store/07pwfg89kj6qmnz18brnsaa16dyqnkrc-gimp-3.0.0-RC2-dev/include/gimp-3.0 -isystem /nix/store/054bzpzh1q9ci74lga47clf10xc6vass-gegl-0.4.52-dev/include/gegl-0.4 -isystem /nix/store/f0s0a5dhl9f03jnk4gmyg0n8m0hz89df-json-glib-1.10.0-dev/include/json-glib-1.0 -isystem /nix/store/6hdwp3pm2xgv4l27sdp92430p4qch1g8-babl-0.1.110-dev/include/babl-0.1 -isystem /nix/store/4xy0x6f6jj3vsw8yw9fzqwr884yq11hv-cairo-1.18.2-dev/include/cairo -isystem /nix/store/dgdy79vrk1pwsi7ncpycdss1byqyxq7g-freetype-2.13.3-dev/include/freetype2 -isystem /nix/store/ji6inyc1hiid4kj2cid3dnrd2qhfmvda-gdk-pixbuf-2.42.12-dev/include/gdk-pixbuf-2.0 -isystem /nix/store/dwvy1la6xcn5r03rfcz8lj698km4w9wb-pango-1.54.0-dev/include/pango-1.0 -isystem /nix/store/w0r1625warqlz1hjpz09harg8hkxhz1v-qtbase-5.15.15-dev/include/QtCore -isystem /nix/store/w0r1625warqlz1hjpz09harg8hkxhz1v-qtbase-5.15.15-dev/./mkspecs/linux-g++ -isystem /nix/store/w0r1625warqlz1hjpz09harg8hkxhz1v-qtbase-5.15.15-dev/include/QtWidgets -isystem /nix/store/w0r1625warqlz1hjpz09harg8hkxhz1v-qtbase-5.15.15-dev/include/QtGui -isystem /nix/store/w0r1625warqlz1hjpz09harg8hkxhz1v-qtbase-5.15.15-dev/include/QtNetwork -DNDEBUG -O3 -std=gnu++11 -fopenmp -pthread -fPIC -MD -MT CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o -MF CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o.d -o CMakeFiles/gmic_gimp_qt.dir/src/Host/Gimp/host_gimp.cpp.o -c /build/gmic-qt/src/Host/Gimp/host_gimp.cpp
In file included from /nix/store/07pwfg89kj6qmnz18brnsaa16dyqnkrc-gimp-3.0.0-RC2-dev/include/gimp-3.0/libgimpbase/gimpbase.h:35,
                 from /nix/store/07pwfg89kj6qmnz18brnsaa16dyqnkrc-gimp-3.0.0-RC2-dev/include/gimp-3.0/libgimp/gimp.h:30,
                 from /build/gmic-qt/src/Host/Gimp/host_gimp.cpp:25:
/nix/store/07pwfg89kj6qmnz18brnsaa16dyqnkrc-gimp-3.0.0-RC2-dev/include/gimp-3.0/libgimpbase/gimpmetadata.h:27:10: fatal error: gexiv2/gexiv2.h: No such file or directory
   27 | #include <gexiv2/gexiv2.h>
      |          ^~~~~~~~~~~~~~~~~
compilation terminated.