br_multicast_is_router takes two arguments when bridge IGMP is enabled
and just one when it's disabled, fix the stub to take two as well.

Fixes: 1a3065a2 ("net: bridge: mcast: prepare is-router function for mcast router split")
Signed-off-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Acked-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: David S. Miller <davem@davemloft.net>