gir: skip glib-enumtypes.h for GObject-2.0. See #267
The newly added GTypes in glib!481 result in g-i generating an enum type
in GLib-2.0 and a GType based one in GObject-2.0.
This makes problems for the dlang bindings (see #267) so skip them for now.
Ideally we should include it in GLib-2.0 like we do with glib-types.h.