compat: disable -Wunused-parameter in win32/headless.c

compat: disable -Wunused-parameter in win32/headless.c

As with the files touched in the previous commit, win32/headless.c does
not include git-compat-util.h, so it doesn't have our UNUSED macro.
Unlike those ones, this is not third-party code, so it would not be a
big deal to modify it.

However, I'm not sure if including git-compat-util.h would create other
headaches (and I don't even have a machine to test this on; I'm relying
on Windows CI to compile it at all). Given how trivial the file is, and
that the unused parameters are not interesting (they are just
boilerplate for the wWinMain() function), we can just use the same trick
as the previous commit and disable the warnings via pragma.

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