posix: sched: Implement sched_rr_get_interval

Implement `sched_rr_get_interval()` POSIX APIs
as a part of PSE53 `_POSIX_PRIORITY_SCHEDULING` option group.
Functions is actually placeholders and just return `ENOSYS`
since Zephyr does not yet support processes or process scheduling.

signed-off-by: Gaetan Perrot <gaetanperrotpro@gmail.com>