unix: fix -Wunused-but-set-variable warning

PR-URL: https://github.com/libuv/libuv/pull/2517
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>
Reviewed-By: Saúl Ibarra Corretgé <saghul@gmail.com>

src/unix/signal.c

@@ -565,6 +565,7 @@ static void uv__signal_stop(uv_signal_t* handle) {
     if (first_oneshot && !rem_oneshot) {
       ret = uv__signal_register_handler(handle->signum, 1);
       assert(ret == 0);
+      (void)ret;
     }
   }