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