pool: Fix compiler warning on mingw64
pool: Fix compiler warning on mingw64

gstpoll.c: In function 'release_event':
gstpoll.c:239:3: error: suggest parentheses around assignment used as
          truth value [-Werror=parentheses]

   if (status = WaitForSingleObject (set->wakeup_event, INFINITE)) {
   ^~

https://bugzilla.gnome.org/show_bug.cgi?id=774108

StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer