background: fix task leak in load_file_async

g_task_run_in_thread takes its own reference to the
task passed in, so we can unref the initial reference.

https://bugzilla.gnome.org/show_bug.cgi?id=696157

src/compositor/meta-background.c

@@ -1129,6 +1129,7 @@ meta_background_load_file_async (MetaBackground          *self,
     g_task_set_task_data (task, task_data, (GDestroyNotify) load_file_task_data_free);
 
     g_task_run_in_thread (task, (GTaskThreadFunc) load_file);
+    g_object_unref (task);
 }
 
 /**