tracing: Export trace_clock_local()

The rcutorture tests need to be able to trace the time of the
beginning of an RCU read-side critical section, and thus need access
to trace_clock_local().  This commit therefore adds a the needed
EXPORT_SYMBOL_GPL().

Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>

kernel/trace/trace_clock.c

diff --git a/kernel/trace/trace_clock.c b/kernel/trace/trace_clock.c
index 3947835..1bbb1b20 100644
--- a/kernel/trace/trace_clock.c
+++ b/kernel/trace/trace_clock.c
@@ -44,6 +44,7 @@
 
 	return clock;
 }
+EXPORT_SYMBOL_GPL(trace_clock_local);
 
 /*
  * trace_clock(): 'between' trace clock. Not completely serialized,