system: hurd: Have the static networking provision 'networking.

Some services require networking, I'm looking at the Guix Build Coordinator
agent.  Networking seems to work in the childhurd VM, so I think this change
makes sense.

* gnu/system/hurd.scm (%base-services/hurd): Add 'networking to the provision
parameter for the static-networking service.

gnu/system/hurd.scm

@@ -79,7 +79,7 @@ (define %base-services/hurd
                  (list (static-networking (interface "lo")
                                           (ip "127.0.0.1")
                                           (requirement '())
-                                          (provision '(loopback))
+                                          (provision '(loopback networking))
                                           (name-servers '("10.0.2.3")))))
         (syslog-service)
         (service guix-service-type