build: Fix includedir in libpeas-gtk pkgconfig Cflags
It should say: $ grep Cflags meson-private/libpeas-gtk-1.0.pc Cflags: -I${includedir}/libpeas-1.0/libpeas-gtk
Not: $ grep Cflags meson-private/libpeas-gtk-1.0.pc Cflags: -I${includedir}/libpeas-gtk-1.0
As there are no headers installed in ${includedir}/libpeas-gtk-1.0