ao_wasapi: Slightly improve timer accuracy

Use QueryPerformanceCounter to improve the accuracy of
IAudioClock::GetPosition.

While this is mainly for "realtime correctness" (usually the delay is a
single sample or less), there are cases where IAudioClock::GetPosition
takes a long time to return from its call (though the documentation doesn't
define what a "long time" is), so correcting its value might be important in
case the documented possible delay happens.