bridge: Fix build error when IGMP_SNOOPING is not enabled

bridge: Fix build error when IGMP_SNOOPING is not enabled

Fix the following build error when IGMP_SNOOPING is not enabled.
In file included from net/bridge/br.c:24:
net/bridge/br_private.h: In function 'br_multicast_is_router':
net/bridge/br_private.h:361: error: 'struct net_bridge' has no member named 'multicast_router'
net/bridge/br_private.h:362: error: 'struct net_bridge' has no member named 'multicast_router'
net/bridge/br_private.h:363: error: 'struct net_bridge' has no member named 'multicast_router_timer'

Signed-off-by: Sridhar Samudrala <[email protected]>
Acked-by: Randy Dunlap <[email protected]>
Acked-by: Herbert Xu <[email protected]>
Signed-off-by: David S. Miller <[email protected]>