userdiff: mark unused parameter in internal callback

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>