Menu|Item: Fix FIXME re non-const interned strings

These are members of the private struct, so it hurts no one to fix this.

gtk/gtkmenuprivate.h

@@ -45,7 +45,7 @@ struct _GtkMenuPrivate
   GtkWidget *old_active_menu_item;
 
   GtkAccelGroup *accel_group;
-  gchar         *accel_path;
+  const char    *accel_path;
 
   GtkMenuPositionFunc position_func;
   gpointer            position_func_data;