php7: add dependency to hash for mysqlnd

php7: add dependency to hash for mysqlnd

The following error shows that mysqlnd depends on functions
provided by hash:

root@OpenWrt:/etc/php7# php-cli -m
PHP Warning:  PHP Startup: Unable to load dynamic library
'mysqlnd.so' (tried: /usr/lib/php/mysqlnd.so (Error
relocating /usr/lib/php/mysqlnd.so: PHP_SHA256Final: symbol
not found), /usr/lib/php/mysqlnd.so.so (Error loading shared
library /usr/lib/php/mysqlnd.so.so: No such file or
directory)) in Unknown on line 0

So let's model this dep in package metadata.

Signed-off-by: Michael Heimpold <mhei@heimpold.de>