src.ctf.fs: make ctf_fs_trace::path an std::string

src.ctf.fs: make ctf_fs_trace::path an std::string

Change-Id: I9938c37f4effca7e83ed2e495feb5dd596d2320e
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8281
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12319
Tested-by: jenkins <jenkins@lttng.org>