(getitem): Change signature

 /* GNU Mailutils -- a suite of utilities for electronic mail
-   Copyright (C) 1999, 2000, 2004 Free Software Foundation, Inc.
+   Copyright (C) 1999, 2000, 2004, 2007 Free Software Foundation, Inc.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Lesser General Public
@@ -39,7 +39,7 @@ struct _mu_iterator
   int (*destroy) (mu_iterator_t itr, void *owner);
   int (*first) (void *owner);
   int (*next) (void *owner);
-  int (*getitem) (void *owner, void **pret);
+  int (*getitem) (void *owner, void **pret, const void **pkey);
   int (*curitem_p) (void *owner, void *item);
   int (*finished_p) (void *owner);
 };