Unbreak HHVM OSS `make install`

Summary: https://reviews.facebook.net/D19959 was in fact wrong in the
way that I commented on there that it was wrong. `make install` totally
doesn't work due to wrong path. Fix.

Fixes https://github.com/facebook/hhvm/issues/3244

Reviewed By: @kmiller68

Differential Revision: D1447093