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>

compat/regex/regex.c

@@ -17,6 +17,8 @@
    License along with the GNU C Library; if not, see
    <http://www.gnu.org/licenses/>.  */
 
+#pragma GCC diagnostic ignored "-Wsign-compare"
+
 #ifdef HAVE_CONFIG_H
 #include "config.h"
 #endif