Fix virNetDevWaitDadFinish stub

Build on non-Linux fails because the virNetDevWaitDadFinish() stub
has unused parameters. Fix by adding appropriate ATTRIBUTE_UNUSED
for these parameters.

Pushing under build-breaker rule.

src/util/virnetdev.c

@@ -1527,7 +1527,8 @@ int virNetDevClearIPAddress(const char *ifname,
 
 /* return after DAD finishes for all known IPv6 addresses or an error */
 int
-virNetDevWaitDadFinish(virSocketAddrPtr *addrs, size_t count)
+virNetDevWaitDadFinish(virSocketAddrPtr *addrs ATTRIBUTE_UNUSED,
+                       size_t count ATTRIBUTE_UNUSED)
 {
     virReportSystemError(ENOSYS, "%s",
                          _("Unable to wait for IPv6 DAD on this platform"));