[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 <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv4/igmp.c

···
  */
 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;