for_each_object: mark unused callback parameters

The for_each_{loose,packed}_object interface uses callback functions,
but not every callback needs all of the parameters. Mark the unused ones
to satisfy -Wunused-parameter.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

packfile.c

@@ -2204,8 +2204,8 @@ int for_each_packed_object(each_packed_object_fn cb, void *data,
 }
 
 static int add_promisor_object(const struct object_id *oid,
-			       struct packed_git *pack,
-			       uint32_t pos,
+			       struct packed_git *pack UNUSED,
+			       uint32_t pos UNUSED,
 			       void *set_)
 {
 	struct oidset *set = set_;