[ticket/11725] Use new paths for phpbb_class_loader in file.php

In the PR #1559, the paths were changed from "{$phpbb_root_path}includes/" to
"{$phpbb_root_path}phpbb/" for the class loader. However, this was not changed
in all files that use it.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11725

PHPBB3-11725