zdtm: It's need add format to fprintf

It's need add format to fprintf to avoid following error in Ubuntu 12.04:
msg.c:59:2: error: format not a string literal and no format arguments [-Werror=format-security]
Signed-off-by: Victor Konyashkin  <vkonyashkin@parallels.com>
Acked-by: Andrey Vagin <avagin@openvz.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

test/zdtm/lib/msg.c

@@ -56,5 +56,5 @@ void test_msg(const char *format, ...)
 	off += vsnprintf(buf + off, sizeof(buf) - off, format, arg);
 	va_end(arg);
 
-	fprintf(stderr, buf);
+	fprintf(stderr, "%s", buf);
 }