Fix: ir: make duplicate event classes check smarter

This patch makes bt_ctf_stream_class_add_event_class()
compare not only event classes pointers when checking
for duplicate, but also equal names and equal IDs.

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