security/zeek: Add @sample for local.zeek

This github issue:

https://github.com/zeek/zeekctl/issues/35

complained about the lack of a local.zeek file on a fresh install;
adding @sample for local.zeek solves this.

Reported by: shadonet

(cherry picked from commit 7c9b2f40c5f2557d87cc1d2ce7d968377b13d6b3)