Compiling on macOS fails due to ffmpeg 5

Building with the default instructions via homebrew on macOS will install ffmpeg 5, `make` will fail with

```
[ 3%] Building CXX object CMakeFiles/decord.dir/src/audio/audio_interface.cc.o
In file included from /Users/ch/git/decord/src/audio/audio_interface.cc:5:
In file included from /Users/ch/git/decord/src/audio/audio_reader.h:10:
In file included from /Users/ch/git/decord/src/audio/../../include/decord/audio_interface.h:9:
/Users/_/decord/src/audio/../../include/decord/../../src/video/ffmpeg/ffmpeg_common.h:187:5: error: unknown type name 'AVBSFContext'; did you mean 'AVIOContext'?
AVBSFContext, Deleterp >;
^~~~~~~~~~~~
AVIOContext
```

The fix is to install ffmpeg 4:

```
brew install cmake ffmpeg@4
brew link ffmpeg@4
```