src.ctf.fs: make ctf_fs_make_port_name return std::string

src.ctf.fs: make ctf_fs_make_port_name return std::string

Change-Id: I9e2f4d2ac47e4f061e5dda1a3813d1a092ffe433
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8296
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12332
Tested-by: jenkins <jenkins@lttng.org>