bird service: add bird to systemPackages

For the tool birdc to monitor and configure bird.

nixos/modules/services/networking/bird.nix

@@ -34,6 +34,7 @@ let
 
       ###### implementation
       config = mkIf cfg.enable {
+        environment.systemPackages = [ pkg ];
         systemd.services.${variant} = {
           description = "BIRD Internet Routing Daemon";
           wantedBy = [ "multi-user.target" ];