refs: mark unused parameters in do_for_each_reflog_helper()

refs: mark unused parameters in do_for_each_reflog_helper()

This is an each_ref_fn callback, so it has to match that interface. We
marked most of these in 63e14ee2d6 (refs: mark unused each_ref_fn
parameters, 2022-08-19), but in this case:

  - this function was created in 31f898397b (refs: drop unused params
    from the reflog iterator callback, 2024-02-21), and most of the
    arguments were correctly mark as UNUSED, but "flags" was missed.

  - commit e8207717f1 (refs: add referent to each_ref_fn, 2024-08-09)
    added a new argument to the each_ref_fn callback. In most callbacks
    it added an UNUSED annotation, but it missed one case.

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