Extend `ArcanistXHPASTLinter::LINT_KEYWORD_CASING`

Summary: Extend `ArcanistXHPASTLinter::LINT_KEYWORD_CASING` to include the `class` keyword.

Test Plan: I can't really added a test case for this without getting "XHP19 Class-Filename Mismatch".

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D10653