package/php: security bump version to 7.4.15
Changelog: https://www.php.net/ChangeLog-7.php#7.4.15
Fixes CVE-2021-21702: http://bugs.php.net/80672
Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>