Gtk: gtk-2.0 compatibility hack

Gtk-2.0 exposes internal implementation fields in .gir, and 'child'
field of Gtk.Bin clashes with lgi's own 'child' mechanism.  Moreover,
offsets to fields are calculated incorrectly in .typelib file for 64b
platforms, therefore it results in crash.

Solve everything by just removing 'child' field from Gtk.Bin

https://github.com/pavouk/lgi/issues/58