Minor change to `src/__phutil_library_init__.php`

Summary: Use `__FUNCTION__` instead of a hardcoded function naming. According to [[http://php.net/manual/en/language.constants.predefined.php | the documentation]], `__FUNCTION__` was added in PHP 4.3.0.

Test Plan: N/A

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9945