ffmpeg: Fix compile error with latest ffmpeg version

Fixes #1164

panda/src/ffmpeg/ffmpegAudioCursor.cxx

@@ -101,7 +101,7 @@ FfmpegAudioCursor(FfmpegAudio *src) :
   _audio_rate = codecpar->sample_rate;
   _audio_channels = codecpar->channels;
 
-  AVCodec *pAudioCodec = avcodec_find_decoder(codecpar->codec_id);
+  const AVCodec *pAudioCodec = avcodec_find_decoder(codecpar->codec_id);
   if (pAudioCodec == nullptr) {
     cleanup();
     return;

panda/src/ffmpeg/ffmpegVideoCursor.cxx

@@ -534,7 +534,7 @@ open_stream() {
   _video_timebase = av_q2d(stream->time_base);
   _min_fseek = (int)(3.0 / _video_timebase);
 
-  AVCodec *pVideoCodec = nullptr;
+  const AVCodec *pVideoCodec = nullptr;
   if (ffmpeg_prefer_libvpx) {
 #if LIBAVCODEC_VERSION_INT >= AV_VERSION_INT(55, 0, 0)
     if (codecpar->codec_id == AV_CODEC_ID_VP9) {