cmdlinenv00 test: fix for clang

clang complains:
> cmdlinenv00.c:35:11: error: comparison of unsigned expression < 0 is always false [-Werror,-Wtautological-compare]
>                 if (ret < 0) {
>                     ~~~ ^ ~

The fix is to use ssize_t not size_t.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

test/zdtm/static/cmdlinenv00.c

@@ -21,7 +21,7 @@ TEST_OPTION(arg3, string, "arg3", 1);
 
 static void read_from_proc(const char *path, char *buf, size_t size)
 {
-	size_t r = 0, ret;
+	ssize_t r = 0, ret;
 	int fd;
 
 	fd = open(path, O_RDONLY);