zabbix: add fping dependency

zabbix: add fping dependency

If fping is not installed on the system the following message is show if
hosts should be monitored via icmp.

Log-Message:
"At least one of '/usr/sbin/fping', '/usr/sbin/fping6' must exist. Both
are missing in the system."

To fix this also, add a dependency to 'fping' for 'zabbix-server' and
'zabbix-proxy' for installation to allow icmp hosts monitoring.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>