Fix: Confusion between stream and stream class IDs

CTF Writer is using the stream id, which is really a "file id",
to populate the packet header. The packet header's id should
match a stream class declaration in the TSDL metadata file.

Reported-by: Rocky Dunlap <rsdunlapiv@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>