Add missing shabang for init file.

I got the following error on ubuntu:

    service[zabbix_agentd] (zabbix::agent_prebuild line 24) had an error: Errno::ENOEXEC: Exec format error - /etc/init.d/zabbix_agentd status

This fixed the issue (thanks to http://brianoneill.blogspot.com/2009/07/exec-format-error-in-cron.html ). :)