net-mgmt/zabbix[4|42]-frontend: Add missing PHP fileinfo extension

This requirement was introduced on Zabbix 4.0.12 and 4.2.6 and it's still
not documented upstream.

PR: 240482
Submitted by: Aleksandr Ignatyev <alex@i.org.ua>
Approved by: maintainer timeout (pg@pakhom.spb.ru, > 2 weeks)