[PATCH] rcu: add missing prototype

[Eric Engestrom]

`make W=1` warns:
kernel/rcu/tree.c:226:15: warning: no previous prototype for ârcu_rnp_online_cpusâ [-Wmissing-prototypes]
 unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp)
               ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Eric Engestrom <eric.engestrom@xxxxxxxxxx>
---

I couldn't figure out how this list is ordered, I'm happy to move this
line if you want :)

---
 kernel/rcu/tree.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index f714f87..23be3dc 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -668,6 +668,7 @@ static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 static bool rcu_nohz_full_cpu(struct rcu_state *rsp);
 static void rcu_dynticks_task_enter(void);
 static void rcu_dynticks_task_exit(void);
+static unsigned long rcu_rnp_online_cpus(struct rcu_node *rnp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
 
-- 
Cheers,
  Eric