[IPV4]: Fix warning in ip_mc_rejoin_group.

Kill warning about unused variable `in_dev' when CONFIG_IP_MULTICAST
is not set.
Signed-off-by: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/igmp.c

@@ -1255,9 +1255,9 @@ out:
  */
 void ip_mc_rejoin_group(struct ip_mc_list *im)
 {
+#ifdef CONFIG_IP_MULTICAST
 	struct in_device *in_dev = im->interface;
 
-#ifdef CONFIG_IP_MULTICAST
 	if (im->multiaddr == IGMP_ALL_HOSTS)
 		return;