WASAPI : Fix Windows <10 build

Author: MarcusTomlinson

RtAudio.cpp

@@ -3837,6 +3837,17 @@ if ( objectPtr )\
 
 typedef HANDLE ( __stdcall *TAvSetMmThreadCharacteristicsPtr )( LPCWSTR TaskName, LPDWORD TaskIndex );
 
+#ifndef __IAudioClient3_INTERFACE_DEFINED__
+MIDL_INTERFACE( "00000000-0000-0000-0000-000000000000" ) IAudioClient3
+{
+  virtual HRESULT GetSharedModeEnginePeriod( WAVEFORMATEX*, UINT32*, UINT32*, UINT32*, UINT32* ) = 0;
+  virtual HRESULT InitializeSharedAudioStream( DWORD, UINT32, WAVEFORMATEX*, LPCGUID ) = 0;
+};
+#ifdef __CRT_UUID_DECL
+__CRT_UUID_DECL( IAudioClient3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 )
+#endif
+#endif
+
 //-----------------------------------------------------------------------------
 
 // WASAPI dictates stream sample rate, format, channel count, and in some cases, buffer size.