fix: Support FFmpeg 5.1.x for decoding (#3816)

For some reason FFmpeg 5.1.x reverted part of the changes made in 5.0.x
on AVCodec.

This fix decoding issues with it.

Ryujinx.Graphics.Nvdec.FFmpeg/Native/AVCodecContext.cs

@@ -9,7 +9,7 @@ namespace Ryujinx.Graphics.Nvdec.FFmpeg.Native
         public unsafe IntPtr AvClass;
         public int LogLevelOffset;
         public int CodecType;
-        public unsafe AVCodecLegacy* Codec;
+        public unsafe AVCodec* Codec;
         public AVCodecID CodecId;
         public uint CodecTag;
         public IntPtr PrivData;