actionmuxer: Make the compiler warn

Warn when the boolean return isn't used, since we
may not initialize the out arguments in the FALSE
case (see the previous commits).

gtk/gtkactionmuxerprivate.h

@@ -73,7 +73,7 @@ gboolean                gtk_action_muxer_query_action                   (GtkActi
                                                                          const GVariantType **parameter_type,
                                                                          const GVariantType **state_type,
                                                                          GVariant           **state_hint,
-                                                                         GVariant           **state);
+                                                                         GVariant           **state) G_GNUC_WARN_UNUSED_RESULT;
 void                    gtk_action_muxer_activate_action                (GtkActionMuxer      *muxer,
                                                                          const char          *action_name,
                                                                          GVariant            *parameter);