libbe: annotate lbh as __unused in be_is_auto_snapshot_name

lbh is included for consistency with other functions and in case
future work needs to use it, but it is currently unused. Mark it,
and a post-OpenZFS-import world will be able to raise WARNS of
libbe to the default (pending some minor changes to openzfs libzfs).

MFC after: 3 days