sched: debug: update exec_clock only when SCHED_DEBUG

micro-optimization: update cfs_rq->exec_clock only if
CONFIG_SCHED_DEBUG=y.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>