bridge: fix compile error when compiling without IPv6 support

Some fields in "struct net_bridge" aren't available when compiling the
kernel without IPv6 support. Therefore adding a check/macro to skip the
complaining code sections in that case.

Introduced by 2cd41431
("bridge: memorize and export selected IGMP/MLD querier port")
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NLinus Lüssing <linus.luessing@web.de>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/bridge/br_multicast.c

@@ -2246,11 +2246,13 @@ bool br_multicast_has_querier_adjacent(struct net_device *dev, int proto)
 		    rcu_dereference(br->ip4_querier.port) == port)
 			goto unlock;
 		break;
+#if IS_ENABLED(CONFIG_IPV6)
 	case ETH_P_IPV6:
 		if (!timer_pending(&br->ip6_other_query.timer) ||
 		    rcu_dereference(br->ip6_querier.port) == port)
 			goto unlock;
 		break;
+#endif
 	default:
 		goto unlock;
 	}