added dependency for selinux module

If selinux is set to enforced, puppet-nagios would need thias-selinux module as dependency. Otherwise it puppet would dump following. 

```
Error: Could not retrieve catalog from remote server: Error 400 on SERVER: Puppet::Parser::AST::Resource failed with error ArgumentError: Invalid resource type selinux::audit2allow at /etc/puppet/environments/production/modules/nagios/manifests/server.pp:685 on node xxx.pythian.com
Warning: Not using cache on failed catalog
Error: Could not retrieve catalog; skipping run
```