Use cppcheck to lint XHPAST source

Summary: Use Cppcheck to lint XHPAST source files.

Test Plan: Ran arc lint --trace support/xhpast/xhpast.cpp and saw cppcheck invoked.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12272