gtk: fix compiler warning with recent glib

https://bugzilla.gnome.org/show_bug.cgi?id=793688
diff --git a/ext/gtk/gstgtkbasesink.c b/ext/gtk/gstgtkbasesink.c
index 843c97f..2f6734f 100644
--- a/ext/gtk/gstgtkbasesink.c
+++ b/ext/gtk/gstgtkbasesink.c
@@ -396,7 +396,7 @@
 
       GST_OBJECT_LOCK (gtk_sink);
       if (gtk_sink->window)
-        window = g_object_ref (gtk_sink->window);
+        window = g_object_ref (GTK_WINDOW (gtk_sink->window));
       GST_OBJECT_UNLOCK (gtk_sink);
 
       if (window)