[prolog] add a hh/1 predicate

Summary:
I was modifying stuff in database_prolog_php.ml for someone else,
use the opportunity to add hh/1

Test Plan:
$ pphp hh.php
...
file('data/users/pad/github/pfff/tests/php/prolog/hh.php', ['data','users','pad','github','pfff','tests','php','prolog','hh.php']).
hh('data/users/pad/github/pfff/tests/php/prolog/hh.php').
kind('test_hh', function).
at('test_hh', 'data/users/pad/github/pfff/tests/php/prolog/hh.php', 3).
arity('test_hh', 0).

Reviewers: jcreed

Reviewed By: jcreed

CC: bryano

Differential Revision: https://phabricator.fb.com/D1090687
Merge branch 'master' of github.com:facebook/pfff