Fix mktime() inside mysql2date() in wp-includes/functions.php

[josephscott]

All of the parameters to mktime() are supposed to be of type int. The problem is that substr() is being used to provide each parameter and it returns a string, not an int. This isn't fatal because PHP will do the conversion for you, but it does through a warning. The simple way to deal with this is to cast all of the substr() calls to int.

Looks good.

(In [5284]) Cast substr()'s to int before passing through mktime() to prevent warning. Props josephscott. fixes #4166