[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>