compat/regex: explicitly ignore "-Wsign-compare" warnings

Explicitly ignore "-Wsign-compare" warnings in our bundled copy of the
regcomp implementation. We don't use the macro introduced in the
preceding commit because this code does not include "git-compat-util.h"
in the first place.

Note that we already directly use "#pragma GCC diagnostic ignored" in
"regcomp.c", so it shouldn't be an issue to use it directly in the new
spot, either.

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