Add xhprof extensoin

[acedude]

It would be awesome if xhprof extension would be added to this buildpack. I'm trying to debug something and it cannot be done without it :/

[sonicoder86]

Any plans on supporting this?
Currently only Blackfire is available for profiling, but it is not fully integrated with Heroku (no SSO, no billing)

[stof]

I'm not maintaining this buildpack anymore, given that the official PHP buildpack maintained by heroku is now much better than at the time this buildpack was started (at that time, the official PHP buildpack was looking like a joke). And @CHH stopped using this buildpack even earlier.