bridge: add empty br_mdb_init() and br_mdb_uninit() definitions.

This patch adds empty br_mdb_init() and br_mdb_uninit() definitions in
br_private.h to avoid build failure when CONFIG_BRIDGE_IGMP_SNOOPING is not set.
These methods were moved from br_multicast.c to br_netlink.c by
commit 3ec8e9f0

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>