test: fix test-spawn compilation

Unbreaks libuv compilation on Windows.

PR-URL: https://github.com/libuv/libuv/pull/1641
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Santiago Gimeno <santiago.gimeno@gmail.com>

test/test-spawn.c

@@ -654,6 +654,9 @@ TEST_IMPL(spawn_tcp_server) {
   struct sockaddr_in addr;
   int fd;
   int r;
+#ifdef _WIN32
+  uv_os_fd_t handle;
+#endif
 
   init_process_options("spawn_tcp_server_helper", exit_cb);