Deal with newer FFmpeg API.

Index: Core/HLE/sceAtrac.cpp
--- Core/HLE/sceAtrac.cpp.orig
+++ Core/HLE/sceAtrac.cpp
@@ -123,6 +123,7 @@ static const int atracDecodeDelay = 2300;
 #ifdef USE_FFMPEG
 
 extern "C" {
+#include "libavcodec/avcodec.h"
 #include "libavformat/avformat.h"
 #include "libswresample/swresample.h"
 #include "libavutil/samplefmt.h"
