compat/win32: fix -Wsign-compare warning in "wWinMain()"

compat/win32: fix -Wsign-compare warning in "wWinMain()"

GCC generates a warning in "headless.c" because we compare `slash` with
`size`, where the former is an `int` and the latter is a `size_t`. Fix
the warning by storing `slash` as a `size_t`, as well.

This commit is being singled out because the file does not include the
"git-compat-util.h" header, and consequently, we cannot easily mark it
with the `DISABLE_SIGN_COMPARE_WARNING` macro.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>