gtype: Add g_type_interface_instantiable_prerequisite()

There is (at most) a single GType that is instantiable and a
prerequisite for an interface. This function returns that type.

This type is necessary in particular when dealing with GValues because a
GValue contains an instance of a type.

docs/reference/gobject/gobject-sections.txt

@@ -65,6 +65,7 @@ g_type_default_interface_unref
 g_type_children
 g_type_interfaces
 g_type_interface_prerequisites
+g_type_interface_instantiable_prerequisite
 g_type_set_qdata
 g_type_get_qdata
 g_type_query