sched: export cpu_clock

the rcutorture module relies on cpu_clock.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/sched_clock.c

@@ -256,3 +256,4 @@ unsigned long long cpu_clock(int cpu)
 
 	return clock;
 }
+EXPORT_SYMBOL_GPL(cpu_clock);