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 3ec8e9f

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

net/bridge/br_private.h

@@ -526,6 +526,12 @@ static inline bool br_multicast_is_router(struct net_bridge *br)
 {
 	return 0;
 }
+static inline void br_mdb_init(void)
+{
+}
+static inline void br_mdb_uninit(void)
+{
+}
 #endif
 
 /* br_netfilter.c */