don't redefine __forceinline with mingw

mingw redefines __forceinline (though I don't think this applies to all
versions of mingw.) Therefore, don't redefine it if mingw has already
defined it.

stb_vorbis.c

@@ -553,7 +553,7 @@ enum STBVorbisError
 #define NULL 0
 #endif
 
-#ifndef _MSC_VER
+#if !defined(_MSC_VER) && !(defined(__MINGW32__) && defined(__forceinline))
    #if __GNUC__
       #define __forceinline inline
    #else