tool_go_sleep: use (void) to spell out we ignore the return value

 *                            | (__| |_| |  _ <| |___

 *                             \___|\___/|_| \_\_____|

 *

 * Copyright (C) 1998 - 2012, Daniel Stenberg, <daniel@haxx.se>, et al.

 * Copyright (C) 1998 - 2014, Daniel Stenberg, <daniel@haxx.se>, et al.

 *

 * This software is licensed as described in the file COPYING, which

 * you should have received as part of this distribution. The terms

#elif defined(WIN32)

  Sleep(ms);

#elif defined(HAVE_POLL_FINE)

  poll((void *)0, 0, (int)ms);

  (void)poll((void *)0, 0, (int)ms);

#else

  struct timeval timeout;

  timeout.tv_sec = ms / 1000L;