Fix: ir: make sure "stream_id" attr is always right

Make sure the "stream_id" attribute of all the event
classes of a given stream class is updated at the following
places:

  * user sets the stream class ID manually: calling
    bt_ctf_stream_class_set_id()
  * stream class ID is automatically set: in
    bt_ctf_trace_add_stream_class()
  * an event class is added to an existing stream
    class: in bt_ctf_stream_class_add_event_class()

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>