Build with newer FFmpeg

Index: Core/HW/SimpleAudioDec.cpp
--- Core/HW/SimpleAudioDec.cpp.orig
+++ Core/HW/SimpleAudioDec.cpp
@@ -118,7 +118,7 @@ class FFmpegAudioDecoder : public AudioDecoder { (priv
 	int channels_;
 
 	AVFrame *frame_ = nullptr;
-	AVCodec *codec_ = nullptr;
+	const AVCodec *codec_ = nullptr;
 	AVCodecContext  *codecCtx_ = nullptr;
 	SwrContext      *swrCtx_ = nullptr;
 
