System: Reset throttler when audio buffers underflow

Should hopefully reduce the chances of the audio source ending up out of
phase from the emulation when vsync is disabled.

src/core/system.h

@@ -154,6 +154,7 @@ void SetThrottleFrequency(float frequency);
 
 /// Updates the throttle period, call when target emulation speed changes.
 void UpdateThrottlePeriod();
+void ResetThrottler();
 
 /// Throttles the system, i.e. sleeps until it's time to execute the next frame.
 void Throttle();