vty: make CPU scheduling optional based on platform support
Add configure checks for sched_* APIs and guard their usage
in cpu_sched_vty with HAVE_SCHED_* macros.
If scheduler functions are not available, return -ENOTSUP
instead of failing at build time.
Also change osmo_cpu_sched_vty_init() to return int.
This improves portability on platforms lacking full sched
support (e.g. non-Linux or restricted environments).
Change-Id: Ic5b7e39fac16531d370cb81f769ba87fef18cb81
