ctf: add weak ref to stream file in index entry

ctf: add weak ref to stream file in index entry

This will be useful to create `bt_msg_iter` without having access to the
`ctf_fs_ds_file_info` struct owning the path to the stream file.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: Ie7173af60330b601bd755822198ed650e3b9b9b4
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1434
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>