[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.

PHPBB3-11725
Merge remote-tracking branch 'marc1706/ticket/11725' into develop

* marc1706/ticket/11725:
  [ticket/11725] Use new paths for phpbb_class_loader in file.php