fsmonitor/win32: mark unused parameter in fsm_os__incompatible()

We never look at the "ipc" argument we receive. It was added in
8f44976882 (fsmonitor: avoid socket location check if using hook,
2022-10-04) to support the darwin fsmonitor code. The win32 code has to
match the same interface, but we should use an annotation to silence
-Wunused-parameter.

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