test-timeline: Ignore the default stage

We need to create the default stage to have the master clock spin, but
we can tell the compiler that the returned value is meaningless.

tests/conform/test-timeline.c

@@ -185,7 +185,7 @@ void
 test_timeline (TestConformSimpleFixture *fixture,
 	       gconstpointer data)
 {
-  ClutterActor *stage;
+  ClutterActor *stage G_GNUC_UNUSED;
   ClutterTimeline *timeline_1;
   TimelineData data_1;
   ClutterTimeline *timeline_2;