Pass in NULL instead of g_cclosure_marshal_generic

NULL is now a shortcut for g_cclosure_marshal_generic, so avoid
referencing it directly.

https://bugzilla.gnome.org/show_bug.cgi?id=654917

gio/gdbusinterfaceskeleton.c

@@ -216,7 +216,7 @@ g_dbus_interface_skeleton_class_init (GDBusInterfaceSkeletonClass *klass)
                   G_STRUCT_OFFSET (GDBusInterfaceSkeletonClass, g_authorize_method),
                   _g_signal_accumulator_false_handled,
                   NULL,
-                  g_cclosure_marshal_generic,
+                  NULL,
                   G_TYPE_BOOLEAN,
                   1,
                   G_TYPE_DBUS_METHOD_INVOCATION);