build: Fix includedir in libpeas-gtk pkgconfig Cflags again
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: 9dbaa55

GNOME/libpeas,GNOME/libpeas