hexagon: add missing cpu.h include

Signed-off-by: Richard Kuo <rkuo@codeaurora.org>

arch/hexagon/kernel/smp.c

 /*
  * SMP support for Hexagon
  *
- * Copyright (c) 2010-2011, Code Aurora Forum. All rights reserved.
+ * Copyright (c) 2010-2012, Code Aurora Forum. All rights reserved.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 2 and
@@ -28,6 +28,7 @@
 #include <linux/sched.h>
 #include <linux/smp.h>
 #include <linux/spinlock.h>
+#include <linux/cpu.h>
 
 #include <asm/time.h>    /*  timer_interrupt  */
 #include <asm/hexagon_vm.h>