zabbix: Always install perl-json for system unit monitoring

Signed-off-by: Florian Pritz <bluewind@xinu.at>

roles/zabbix-agent/tasks/main.yml

 ---
 
 - name: install packages
-  pacman: name=zabbix-agent,python-dbus
+  pacman: name=zabbix-agent,python-dbus,perl-json
 
 - name: install packages for borg monitoring
   pacman: name=perl-json