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

lib/libbe/be.c

@@ -563,7 +563,7 @@ be_setup_snapshot_name(libbe_handle_t *lbh, char *buf, size_t buflen)
 }
 
 bool
-be_is_auto_snapshot_name(libbe_handle_t *lbh, const char *name)
+be_is_auto_snapshot_name(libbe_handle_t *lbh __unused, const char *name)
 {
 	const char *snap;
 	int day, hour, minute, month, second, serial, year;