php: security bump to version 5.5.21

Fixes:
CVE-2015-0231 - Use After Free Vulnerability in PHP's unserialize()
CVE-2014-9427 - Out of bounds read crashes php-cgi
CVE-2015-0232 - Free called on unitialized pointer

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/php/php.hash

@@ -1,2 +1,2 @@
 # From http://php.net/downloads.php
-md5	51377838dfee2d7bb4715674ac715394	php-5.5.20.tar.xz
+md5	79664ce44f7c93f355a25a3fe3dcc91b	php-5.5.21.tar.xz

package/php/php.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-PHP_VERSION = 5.5.20
+PHP_VERSION = 5.5.21
 PHP_SITE = http://www.php.net/distributions
 PHP_SOURCE = php-$(PHP_VERSION).tar.xz
 PHP_INSTALL_STAGING = YES