groups.c test: fix for clang

clang complains:
> clang -g -O2 -Wall -Werror -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0  -iquote ../lib/arch/x86/include -I../lib   groups.c ../lib/libzdtmtst.a ../lib/libzdtmtst.a -o groups
> groups.c:22:9: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
>         if (ng < 0) {
>             ~~ ^ ~

Declare ng as int.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/groups.c

@@ -14,7 +14,8 @@ const char *test_author	= "Pavel Emelianov <xemul@parallels.com>";
 
 int main(int argc, char **argv)
 {
-	unsigned int ng, *grp, *grp2, i, max;
+	int ng;
+	unsigned int *grp, *grp2, i, max;
 
 	test_init(argc, argv);