video_core: constify AVCodec for ffmpeg >= 5.0

src/video_core/command_classes/codecs/codec.cpp:177:16: error: assigning to 'AVCodec *' from 'const AVCodec *' discards qualifiers
    av_codec = avcodec_find_decoder(codec);
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~

src/video_core/command_classes/codecs/codec.h

@@ -66,7 +66,7 @@ private:
     bool initialized{};
     NvdecCommon::VideoCodec current_codec{NvdecCommon::VideoCodec::None};
 
-    AVCodec* av_codec{nullptr};
+    const AVCodec* av_codec{nullptr};
     AVCodecContext* av_codec_ctx{nullptr};
     AVBufferRef* av_gpu_decoder{nullptr};