php: security bump to version 5.6.21

Fixes (CVEs not assigned yet):
bug #72094 - Out of bounds heap read access in exif header processing
bug #71912 - libgd: signedness vulnerability
bug #72061 - Out-of-bounds reads in zif_grapheme_stripos with negative offset
bug #71843 - null ptr deref ZEND_RETURN_SPEC_CONST_HANDLER
bug #71952 - Corruption inside imageaffinematrixget

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
-sha256	2b87d40213361112af49157a435e0d4cdfd334c9b7c731c8b844932b1f444e7a	php-5.6.20.tar.xz
+sha256	566ff1a486cb0485ed477a91ea292423f77a58671270ff73b74e67e3ce7084f9	php-5.6.21.tar.xz

package/php/php.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-PHP_VERSION = 5.6.20
+PHP_VERSION = 5.6.21
 PHP_SITE = http://www.php.net/distributions
 PHP_SOURCE = php-$(PHP_VERSION).tar.xz
 PHP_INSTALL_STAGING = YES