Fix visibility of the ArcanistPhpcsLinter::getMandatoryFlags method

Summary: Ref T6822.

Test Plan: All the cool linters are doing it.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6822

Differential Revision: https://secure.phabricator.com/D11414