bridge: mcast: fix br_multicast_dev_del warn when igmp snooping is not defined

Fix:
net/bridge/br_if.c: In function 'br_dev_delete':
>> net/bridge/br_if.c:284:2: error: implicit declaration of function
>> 'br_multicast_dev_del' [-Werror=implicit-function-declaration]
     br_multicast_dev_del(br);
     ^
   cc1: some warnings being treated as errors

when igmp snooping is not defined.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>