workqueue: move function definitions within CONFIG_SMP block

In commit 7ee681b2 ("workqueue: Convert to state machine callbacks"),
three new function definitions were added: ‘workqueue_prepare_cpu’,
‘workqueue_online_cpu’ and ‘workqueue_offline_cpu’.

Move these function definitions within a CONFIG_SMP block since they are
not used outside of it. This will match function declarations in header
<include/linux/workqueue.h>, and silence the following gcc warning (W=1):

  kernel/workqueue.c:4743:5: warning: no previous prototype for ‘workqueue_prepare_cpu’ [-Wmissing-prototypes]
  kernel/workqueue.c:4756:5: warning: no previous prototype for ‘workqueue_online_cpu’ [-Wmissing-prototypes]
  kernel/workqueue.c:4783:5: warning: no previous prototype for ‘workqueue_offline_cpu’ [-Wmissing-prototypes]
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -4626,6 +4626,8 @@ void wq_worker_comm(char *buf, size_t size, struct task_struct *task)
 	mutex_unlock(&wq_pool_attach_mutex);
 }
 
+#ifdef CONFIG_SMP
+
 /*
  * CPU hotplug.
  *
@@ -4846,8 +4848,6 @@ int workqueue_offline_cpu(unsigned int cpu)
 	return 0;
 }
 
-#ifdef CONFIG_SMP
-
 struct work_for_cpu {
 	struct work_struct work;
 	long (*fn)(void *);