Added explicit type-cast to hash_pbkdf2

It seems HHVM does not like the use of
dynamic typing  in hash_pbkdf2() (also it's
kind of not the best idea in general), so
added explicit (int) casting to parameters.

Follows-up: I0a9c972931a0eff0cfb2619cef3ddffd03710285
Change-Id: I8adae3cc2cdf8029be59e2d62a3ab0fbfb73b7aa
Merge "Added explicit type-cast to hash_pbkdf2"