ir: make sure you can't add a SC to a trace with a native BO

ir: make sure you can't add a SC to a trace with a native BO

When you create a trace object with bt_ctf_trace_create(), its native
byte order is set to BT_CTF_BYTE_ORDER_NATIVE, which essentially means
"unset". Before freezing the trace (which only happens in
bt_ctf_trace_add_stream_class()), make sure that the trace's native byte
order is a real byte order, not still BT_CTF_BYTE_ORDER_NATIVE (unset).

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