compat: mark unused parameters in win32/mingw functions

compat: mark unused parameters in win32/mingw functions

The compat/ directory contains many stub functions, wrappers, and so on
that have to conform to a specific interface, but don't necessarily need
to use all of their parameters. Let's mark them to avoid complaints from
-Wunused-parameter.

This was done mostly via guess-and-check with the Windows build in
GitHub CI. I also confirmed that the win+VS build is similarly happy.

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