the trace class \p trace_class (see
bt_ctf_trace_get_stream_class_count()).
@postrefcountsame{trace_class}
-@postsuccessrefcountretinc
@sa bt_ctf_trace_get_stream_class_by_id(): Finds a stream class by ID.
@sa bt_ctf_trace_add_stream_class(): Adds a stream class to a trace class.