glib: fix FindClassProperty()'s copy-paste error

The implementation of FindClassProperty() was the same as the
implementation of FindInterfaceProperty(), both receiving an iface
instead of a GObjectClass* [1].

The reason of this oversight can well be explained by the fact that
FindClassProperty() is private and not used, actually. However, we
may need it soonish as a good bind to g_object_class_find_property.

[1] https://developer.gnome.org/gobject/unstable/gobject-The-Base-Object-Type.html#g-object-class-find-property