avformat: fix AVStream private fields marker

avformat: fix AVStream private fields marker

Public fields were added after the private fields (negating the entire
point of this). New private fields go into AVStreamInternal anyway.

The new marker was set by guessing which fields are supposed to be
private and wshich not. recommended_encoder_configuration is accessed by
ffserver_config.c directly, and is supposed to use the public API.

ffmpeg.c accesses AVStream.cur_dts, even though it's a private field,
but that seems to be an older error.