x86: enable irq in default_idle on 64-bit

local_irq_enable() is missing after sched_clock_idle_wakeup_event().

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>