Enable hhvm hotprofiler

I asked brett the other day about hotprofiler at lunch, and he
indicated that having it compiled in is standard procedure.

When hotprofiler is disabled xhprof_disable() always returns null,
with it enabled we get full profiling data.

Change-Id: Icaf42b2efcc0fac7e6b9faa99f8d81fa9c602229