[PATCH] build warning fix

This patch has been verified to apply cleanly to 2.5.30

This patch fixes a build warning in smp.h.  register_cpu_notifier uses
struct notifier_block in its argument list.  Unfortunately, there are
places where smp.h is included before the definition of this structure.

include/linux/smp.h

@@ -100,6 +100,8 @@ static inline void smp_send_reschedule_all(void) { }
 #define per_cpu(var, cpu)			var
 #define this_cpu(var)				var
 
+struct notifier_block;
+
 /* Need to know about CPUs going up/down? */
 static inline int register_cpu_notifier(struct notifier_block *nb)
 {