lang/php56: update 5.6.20 -> 5.6.21

Changelog: http://www.php.net/ChangeLog-5.php#5.6.21

PR: 209145
Reported by Christian Schwarz <me@cschwarz.com>
Security: CVE-2016-3074
Security: https://vuxml.FreeBSD.org/freebsd/5764c634-10d2-11e6-94fa-002590263bf5.html