Deal with newer FFmpeg API.

Index: Core/AVIDump.cpp
--- Core/AVIDump.cpp.orig
+++ Core/AVIDump.cpp
@@ -91,7 +91,7 @@ bool AVIDump::Start(int w, int h)
 
 bool AVIDump::CreateAVI() {
 #ifdef USE_FFMPEG
-	AVCodec *codec = nullptr;
+	const AVCodec *codec = nullptr;
 
 	// Use gameID_EmulatedTimestamp for filename
 	std::string discID = g_paramSFO.GetDiscID();
