userdiff: mark unused parameter in internal callback

Since f12fa9ee6c (userdiff: add and use for_each_userdiff_driver(),
2021-04-08), lookup of userdiffs is done with a generic
for_each_userdiff_driver(). But the name lookup doesn't use the "type"
field, of course.

We can't get rid of that field from the generic interface because it is
used by t/helper/test-userdiff.c. So mark it as unused in this instance
to silence -Wunused-parameter.

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

userdiff.c

@@ -315,7 +315,8 @@ struct find_by_namelen_data {
 };
 
 static int userdiff_find_by_namelen_cb(struct userdiff_driver *driver,
-				       enum userdiff_driver_type type, void *priv)
+				       enum userdiff_driver_type type UNUSED,
+				       void *priv)
 {
 	struct find_by_namelen_data *cb_data = priv;