Fix starting the firewall

Probably as a result of 992c514a20, it
was not being started anymore.

My understanding of systemd.special(7) (section "Special passive
system units") is that the firewall should want network-pre.target,
rather than the other way around (not very intuitive...). This in
itself does not cause the firewall to be wanted, which is why the
wanted-by relationship with multi-user.target is necessary.

http://hydra.nixos.org/build/39965589

nixos/modules/services/networking/firewall.nix

@@ -490,7 +490,8 @@ in
 
     systemd.services.firewall = {
       description = "Firewall";
-      wantedBy = [ "network-pre.target" ];
+      wantedBy = [ "multi-user.target" ];
+      wants = [ "network-pre.target" ];
       before = [ "network-pre.target" ];
       after = [ "systemd-modules-load.service" ];