mark unused parameters in trivial compat functions

mark unused parameters in trivial compat functions

When a platform feature isn't available or in use, we sometimes
conditionally compile empty or trivial functions to turn these into
noops. We need to annotate their parameters so that -Wunused-parameters
won't complain about them.

Note that there are many more of these in compat/mingw.h, but we'll
leave them for now, as there's some trickery required to get the UNUSED
macro available there.

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