Fix invalidating OPcache

Somehow the argument to opcache_invalidate() got messed up.

https://github.com/phpbb/phpbb/commit/bd12427fd447dd4a3be89e8aa1656d0cbe7cc04a#diff-19a7a759abd351432d2d434ee31929faR573

https://github.com/phpbb/phpbb/commit/b7deef46ac2f49b310d05d94067607e8f19c0817#diff-19a7a759abd351432d2d434ee31929faR579

$this->cache_file is the correct argument in functions_messenger.php:process, maybe it got mixed up. It should be changed back to $file.