rcu: Fix sparse warning about rcu_batches_completed_preempt() being non-static

fix sparse warning about rcu_batches_completed_preempt() being non-static by
marking it as static

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree_plugin.h

@@ -134,7 +134,7 @@ static void __init rcu_bootup_announce(void)
  * Return the number of RCU-preempt batches processed thus far
  * for debug and statistics.
  */
-long rcu_batches_completed_preempt(void)
+static long rcu_batches_completed_preempt(void)
 {
 	return rcu_preempt_state.completed;
 }