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

This fixes an ia64 build bug reported by Tony Luck.

Reported-by: Tony Luck <tony.luck@gmail.com>
Signed-off-by: Clark Williams <clark.williams@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1361373550-4011-2-git-send-email-clark.williams@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>