Add UserBufferManagementInit()

Used to reset any state required in UserBufferManagement(). Weak default
implementation provided, which does nothing.

Also tidied up weak default implementation of UserBufferManagement(),
and added a header file to define both function protypes.

module_usb_audio/audio_io/xua_audio.h

@@ -1,6 +1,8 @@
 #ifndef __audio_h__
 #define __audio_h__
 
+#if __XC__
+
 #include "dfu_interface.h"
 
 typedef interface audManage_if
@@ -46,4 +48,6 @@ void audio(chanend c_in,
 
 void SpdifTxWrapper(chanend c_spdif_tx);
 
+#endif // __XC__
+
 #endif // __audio_h__