compat/mmap: mark unused argument in git_munmap()

compat/mmap: mark unused argument in git_munmap()

Our mmap compat code emulates mapping by using malloc/free. Our
git_munmap() must take a "length" parameter to match the interface of
munmap(), but we don't use it (it is up to the allocator to know how big
the block is in free()).

Let's mark it as UNUSED to avoid complaints from -Wunused-parameter.
Otherwise you cannot build with "make DEVELOPER=1 NO_MMAP=1".

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