Fix vgm-audio failing to compile on Clang on Windows

On Windows, audio/CMakeLists.txt (AKA vgm-audio) adds libs/include_vc to
the include path on MSVC, and libs/include_mingw on all other
compilers. This works on MSVC and MinGW, but causes compilation errors
on Clang on Windows (msvc-abi), which uses MSVC's headers. Since
vgm-audio adds libs/include_mingw to Clang's include path, MSVC's
headers include libs/include_mingw/sal.h instead of Microsoft's sal.h,
and fail to compile (due to missing _In_ etc.).

We cannot switch Clang to use include_vc, since Clang hits
-Winvalid-token-paste errors when compiling libs/include_vc/XAudio2.h.

Instead, fix this issue by keeping Clang on libs/include_mingw, but
renaming include_mingw/sal.h to sal_stub.h.

Author: nyanpasu64

libs/include_mingw/XAudio2.h

@@ -62,7 +62,7 @@ DEFINE_IID(IXAudio2, 8bcf1f58, 9fe7, 4583, 8a, c6, e2, ad, c4, 65, c8, bb);
     #include <objbase.h>    // Windows COM declarations
 #endif
 
-#include <sal.h>            // Markers for documenting API semantics
+#include "sal_stub.h"       // Markers for documenting API semantics
 #include <audiodefs.h>      // Basic audio data types and constants
 #include <xma2defs.h>       // Data types and constants for XMA2 audio
 

libs/include_mingw/sal.h renamed to libs/include_mingw/sal_stub.h

@@ -10,7 +10,7 @@
 #ifndef __XMA2DEFS_INCLUDED__
 #define __XMA2DEFS_INCLUDED__
 
-#include <sal.h>        // Markers for documenting API semantics
+#include "sal_stub.h"   // Markers for documenting API semantics
 #include <winerror.h>   // For S_OK, E_FAIL
 #include <audiodefs.h>  // Basic data types and constants for audio work