clockevents: unexport tick_nohz_get_sleep_length

This patch removes the unused 
EXPORT_SYMBOL_GPL(tick_nohz_get_sleep_length).

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

kernel/time/tick-sched.c

@@ -320,8 +320,6 @@ ktime_t tick_nohz_get_sleep_length(void)
 	return ts->sleep_length;
 }
 
-EXPORT_SYMBOL_GPL(tick_nohz_get_sleep_length);
-
 /**
  * nohz_restart_sched_tick - restart the idle tick from the idle task
  *