fsmonitor: mark unused hashmap callback parameters

fsmonitor: mark unused hashmap callback parameters

Like many hashmap comparison functions, our cookies_cmp() does not look
at its extra void data parameter. This should have been annotated in
02c3c59e62 (hashmap: mark unused callback parameters, 2022-08-19), but
this new case was added around the same time (plus fsmonitor is not
built at all on Linux, so it is easy to miss there).

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