if IPV6_TCLASS isn't defined do something reasonable

src/iperf_api.c

@@ -1388,10 +1388,15 @@ iperf_init_stream(struct iperf_stream *sp, struct iperf_test *test)
     /* Set IP TOS */
     if ((opt = test->settings->tos)) {
         if (test->settings->domain == AF_INET6) {
+#ifdef IPV6_TCLASS
             if (setsockopt(sp->socket, IPPROTO_IPV6, IPV6_TCLASS, &opt, sizeof(opt)) < 0) {
                 i_errno = IESETCOS;
                 return (-1);
             }
+#else
+        i_errno = IESETCOS;
+        return (-1);
+#endif
         } else {
             if (setsockopt(sp->socket, IPPROTO_IP, IP_TOS, &opt, sizeof(opt)) < 0) {
                 i_errno = IESETTOS;