gi/pygobject-external.h: Remove GCCism
gi/pygobject-external.h: Remove GCCism

We can use G_GNUC_UNUSED from GLib that does the same purpose of
__attribute__ ( (unused)) when we are using a GCC that provides this.

The GLib headers that defines that macro should have already been included
before this header is included (due to use of gboolean).

https://bugzilla.gnome.org/show_bug.cgi?id=692856

davibe/pygobject,GNOME/pygobject,GNOME/pygobject,davibe/pygobject,davibe/pygobject,GNOME/pygobject,davibe/pygobject,pexip/pygobject,pexip/pygobject,pexip/pygobject,sfeltman/pygobject,sfeltman/pygobject,sfeltman/pygobject