gi: fix build using glib as subproject

gobject-introspection fails to build with newer versions of meson since it's using file directly from a subproject rather than getting them through a variable.

gobject-introspection| subprojects\gobject-introspection\gir\meson.build:347:0: Exception: Sandbox violation: Tried to grab file gmodule.h from a nested subproject.

The following MR uses variables that are added in a new MR to GLib.

What I don't know if it's possible to get these file directly from existing variables by filtering by name for example.

Requires: glib!2465 (merged)

CC: @nirbheek