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: NPaul E. McKenney <paul.mckenney@linaro.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>