sched: export cpu_clock

the rcutorture module relies on cpu_clock.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched_clock.c b/kernel/sched_clock.c
index 3c696db..ed5a8c4 100644
--- a/kernel/sched_clock.c
+++ b/kernel/sched_clock.c
@@ -256,3 +256,4 @@
 
 	return clock;
 }
+EXPORT_SYMBOL_GPL(cpu_clock);