Statically link xhpast.exe

Summary:
Some users reported that this virus doesn't always work.
Re-compiled by adding -static to xhpast in Makefile.

Test Plan:
Renamed MinGW directory which previously triggered the problem. Then:

$ xhpast.exe < xhpast_parse.php
{"tree":[9000, 0, 291, [...]]}

Reviewers: epriestley, btrahan

Reviewed By: btrahan

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3950