interactive/touch-events: Add a missing return value

The draw_touches() function must return a boolean value.

https://bugzilla.gnome.org/show_bug.cgi?id=678391

tests/interactive/test-touch-events.c

@@ -75,6 +75,8 @@ draw_touches (ClutterCairoTexture *canvas,
               cairo_t             *cr)
 {
   g_slist_foreach (events, (GFunc) draw_touch, cr);
+
+  return TRUE;
 }
 
 static gboolean