nixos: add fail2ban to module-list.nix

Now that the fail2ban service has the ".enable" option, I think it's
time to add it to the module list, so that we can enable it in
configuration.nix like this:

  services.fail2ban.enable = true;

nixos/modules/module-list.nix

@@ -257,6 +257,7 @@
   ./services/search/elasticsearch.nix
   ./services/search/solr.nix
   ./services/security/clamav.nix
+  ./services/security/fail2ban.nix
   ./services/security/fprot.nix
   ./services/security/frandom.nix
   ./services/security/haveged.nix