Fix outdated calling convention in SAASound (maybe fix MinGW)

https://docs.microsoft.com/en-us/cpp/cpp/stdcall?view=msvc-170

> For compatibility with previous versions, _stdcall is a synonym for __stdcall
> unless compiler option /Za (Disable language extensions) is specified.

extern/SAASound/src/SAASound.h

@@ -53,7 +53,7 @@ typedef unsigned long SAAPARAM;
 #endif
 
 #ifdef _WIN32
-#define SAAAPI _stdcall
+#define SAAAPI __stdcall
 #else
 #define SAAAPI
 #endif

extern/SAASound/src/types.h

@@ -30,7 +30,7 @@ typedef struct
 } ENVDATA;
 
 #ifdef WIN32
-extern "C" void _stdcall OutputDebugStringA (char*);
+extern "C" void __stdcall OutputDebugStringA (char*);
 #endif
 
 #endif