webserver::php5-mysql: use require_package()

Factored out of I69d4a2821. Bryan's iegreview patch needs to php5-mysql
package. If it declares it, it will conflict with other classes applied on
zirconium, which include webserver::php5-mysql. The iegreview class should not
include webserver::php5-mysql itself, though, because that class is deprecated.

Change-Id: Ieb3c07bb4a963fa2a32086efbb11e5493435cf39