ZP-1187 Mail/sendmail.php: update header

Small part of upstream commit b84fcee: make code php5+ only.
This was committed so the next commit would apply unchanged.

Released under the Affero GNU General Public License (AGPL) version 3.

src/backend/imap/Mail/sendmail.php

 <?php
 //
 // +----------------------------------------------------------------------+
-// | PHP Version 4                                                        |
+// | PHP Version 5                                                        |
 // +----------------------------------------------------------------------+
 // | Copyright (c) 1997-2003 The PHP Group                                |
 // +----------------------------------------------------------------------+