workspace: Respect the not_this_one parameter passed in

In the case of focus-follows-mouse, we need to ensure that we
do not select a certain window after closing another one.

https://bugzilla.gnome.org/show_bug.cgi?id=675982

src/core/workspace.c

@@ -1312,7 +1312,7 @@ focus_ancestor_or_top_window (MetaWorkspace *workspace,
 
   window = meta_stack_get_default_focus_window (workspace->screen->stack,
                                                 workspace,
-                                                NULL);
+                                                not_this_one);
 
   if (window)
     {