Fix a bison deprecation warning

Summary:
Currently, running make parser produces the following warning:

parser.y:72.1-14: warning: deprecated directive, use ‘%name-prefix’ [-Wdeprecated]
 %name-prefix = "xhpast"
  ^^^^^^^^^^^^^^

Test Plan: One less warning.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13972