src.ctf.fs: move ctf_fs_file to file.hpp

This helps resolve some cyclic include problem between file.hpp and
fs.hpp in the next patches (and I think it makes sense to put it there).

Change-Id: I2904d17eb3ea51699464fba17a992ddf3b691f60
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/8259
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12297