Minor change to src/__phutil_library_init__.php

Summary: Use __FUNCTION__ instead of a hardcoded function naming. According to 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