Windows: Fix race between windows_clock_gettime_threaded() and windows_clock_gettime()

Windows: Fix race between windows_clock_gettime_threaded() and windows_clock_gettime()

* When the timer thread is created in windows_init(), it performs an
  initialisation phase in which it uses QueryPerformanceFrequency() to determine
  if there is a high resolution timer available, and sets hires_frequency and
  hires_ticks_to_ps appropriately. However, since windows_init() does not wait for
  this initialisation phase to complete, windows_clock_gettime() can be called
  before hires_frequency and hires_ticks_to_ps have been updated. This can result
  in windows_clock_gettime() temporarily returning real-time clock values even
  though the platform supports a monotonic clock.
* See http://sourceforge.net/mailarchive/forum.php?thread_name=1373620013-3574-1-git-send-email-simon.haggett%40realvnc.com&forum_name=libusbx-devel