build: Fix includedir in libpeas-gtk pkgconfig Cflags again
The PeasGtk-1.0.gir refers to <c:include name="libpeas-gtk/peas-gtk.h"/>, so the Cflags should be the same as in libpeas-1.0.pc: --Cflags: -I${includedir}/libpeas-1.0
Fixes: 9dbaa554
The PeasGtk-1.0.gir refers to <c:include name="libpeas-gtk/peas-gtk.h"/>, so the Cflags should be the same as in libpeas-1.0.pc: --Cflags: -I${includedir}/libpeas-1.0
Fixes: 9dbaa554