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

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

Change-Id: Ifbe7cd2c184031465428c28701131a82ba219b02
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8265
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12303
Tested-by: jenkins <jenkins@lttng.org>