fail2ban service: add iproute to PATH

iproute is required for blocking via null routes; without it, rules
based on routes.conf will fail.

Closes #15638

nixos/modules/services/security/fail2ban.nix

···
         partOf = optional config.networking.firewall.enable "firewall.service";
 
         restartTriggers = [ fail2banConf jailConf ];
-        path = [ pkgs.fail2ban pkgs.iptables ];
+        path = [ pkgs.fail2ban pkgs.iptables pkgs.iproute ];
 
         preStart =
           ''