bridge: adding stubs for multicast exports

To make users (e.g. batman-adv soon) load- and runnable even if the
bridge was compiled without snooping capabilities - or even if the
kernel was compiled without any bridge code at all.

Signed-off-by: Linus Lüssing <linus.luessing@web.de>
Signed-off-by: David S. Miller <davem@davemloft.net>