This needs to be explicitly specified since 3a67da38 relocated these methods into Gdk.Pixbuf, which is underneath an element that specified `library="Library.GdkPixbuf"`
This fixes runtime errors when calling this method (similar for gdk_pixbuf_get_from_window)
This exposes gdk_pixbuf_get_from_window and gdk_pixbuf_get_from_surface via Gdk.Pixbuf constructors. Previously they were in a hidden GdkPixbuf_ class.
Fixes: #174