sched: move RR_TIMESLICE from sysctl.h to rt.h

Originally submitted by Clark Williams as part of a cleanup,
but happens also to fix an ia64 build problem:

arch/ia64/kernel/init_task.c:38: error: 'RR_TIMESLICE' undeclared here (not in a function)

Signed-off-by: Clark Williams <clark.williams@gmail.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>