Linter fixes for XHPAST source

Summary: See D12272#121803.

Test Plan: Admittedly, I don't know much about C/C++, so this could likely be wrong. I ran arc unit --everything to verify that nothing was majorly broken.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12273