bridge: Add missing policy entry for IFLA_BRPORT_FAST_LEAVE

Fixes: c2d3babf

 ("bridge: implement multicast fast leave")
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>