Don't use g_slist_next in gtkstock.c

We generally use ->next directly.

gtk/deprecated/gtkstock.c

@@ -255,7 +255,7 @@ gtk_stock_list_ids (void)
     {
       GList *next;
 
-      next = g_list_next (ids);
+      next = ids->next;
 
       if (last_id && strcmp (ids->data, last_id) == 0)
         {
@@ -268,7 +268,7 @@ gtk_stock_list_ids (void)
         }
 
       g_list_free_1 (ids);
-      
+
       ids = next;
     }