Fix deadlock in threaded resolver

When you're using the threaded resolver and using a sync call
without a cancellable the resolve_sync forgot to unlock the
initial req->mutex lock, leading to a deadlock when unrefing
the request.

gio/gthreadedresolver.c

@@ -319,6 +319,7 @@ resolve_sync (GThreadedResolver         *gtr,
   if (!req->cancellable || !gtr->thread_pool)
     {
       req->resolve_func (req, error);
+      g_mutex_unlock (req->mutex);
       return;
     }