net: bridge: mcast: fix stub definition of br_multicast_querier_exists

net: bridge: mcast: fix stub definition of br_multicast_querier_exists

The commit cited below has changed only the functional prototype of
br_multicast_querier_exists, but forgot to do that for the stub
prototype (the one where CONFIG_BRIDGE_IGMP_SNOOPING is disabled).

Fixes: 8d33d22394b5 ("net: bridge: mcast: add support for raw L2 multicast groups")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20201101000845.190009-1-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>