avcodec: do not use field removed in API 60
AVCodecContext.thread_safe_callbacks will be removed in API 60:
Thread-unsafe get_buffer2() implementations will be invalid starting with LIBAVCODEC_VERSION_MAJOR=60; in other words, libavcodec will behave as if this field was always set to 1.
In current FFmpeg 5.0 (API 59), we must still set the variable, and it is already deprecated, so we can't get rid of the deprecation warning.