bridge: mcast: Expose br_multicast_new_group_src()

Currently, new group source entries are only created in response to
received Membership Reports. Subsequent patches are going to allow user
space to install (*, G) entries with a source list.

As a preparatory step, expose br_multicast_new_group_src() so that it
could later be invoked from the MDB code (i.e., br_mdb.c) that handles
RTM_NEWMDB messages.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Acked-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>