fdtable: workaround posix->zephyr->posix dependency cycle

Currently Zephyr depends on POSIX, which presents a
dependency cycle.

Newlib does not seem to like this dependency cycle so it
requires that we re-declare off_t and ssize_t.

We should really stop using off_t and ssize_t below the
POSIX API line in Zephyr.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>