services: iptables: Provide a default value.

There doesn't seem to be a reason to force users to write

(service iptables-service-type
         (iptables-configuration))

instead of simply

(service iptables-service-type)

This patch provides a default value for the iptables-service-type.

* gnu/services/networking.scm (iptables-service-type): Set default-value.

Change-Id: I93b6c544dfb064c7a0a999549dff61007a38f842
Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/services/networking.scm

@@ -2110,6 +2110,7 @@ COMMIT
 (define iptables-service-type
   (service-type
    (name 'iptables)
+   (default-value (iptables-configuration))
    (description
     "Run @command{iptables-restore}, setting up the specified rules.")
    (extensions