Fix build with gcc 4.5

Based on patch by Dominique Leuenberger
<dominique-gnomezilla@leuenberger.net>

https://bugzilla.gnome.org/show_bug.cgi?id=606719

src/core/core.c

@@ -526,6 +526,9 @@ meta_core_get_menu_accelerator (MetaMenuOp           menu_op,
   
   switch (menu_op)
     {
+    case META_MENU_OP_NONE:
+      name = "none";
+      break;
     case META_MENU_OP_DELETE:
       name = "close";
       break;