Remove dependency of AudioIO on Scrubbing.cpp ...

... though the demotion of the constant into AudioIO.h isn't wholly satisfactory
and there is still the scrubbing queue in AudioIO

src/AudioIOBase.h

@@ -16,6 +16,7 @@ Paul Licameli split from AudioIO.h
 
 #include <atomic>
 #include <cfloat>
+#include <functional>
 #include <memory>
 #include <vector>
 #include <wx/string.h>
@@ -102,6 +103,11 @@ struct AudioIOStartStreamOptions
 
    // contents may get swapped with empty vector
    PRCrossfadeData      *pCrossfadeData{};
+
+   // An unfortunate thing needed just to make scrubbing work on Linux when
+   // we can't use a separate polling thread.
+   // The return value is a number of milliseconds to sleep before calling again
+   std::function< unsigned long() > playbackStreamPrimer;
 };
 
 ///\brief A singleton object supporting queries of the state of any active