avr32: Minor pm_power_off cleanup

Include <linux/pm.h> to see the declaration of pm_power_off, and remove
unneeded NULL initializer.
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>

arch/avr32/kernel/process.c

@@ -9,6 +9,7 @@
 #include <linux/module.h>
 #include <linux/kallsyms.h>
 #include <linux/fs.h>
+#include <linux/pm.h>
 #include <linux/ptrace.h>
 #include <linux/reboot.h>
 #include <linux/tick.h>
@@ -20,7 +21,7 @@
 
 #include <mach/pm.h>
 
-void (*pm_power_off)(void) = NULL;
+void (*pm_power_off)(void);
 EXPORT_SYMBOL(pm_power_off);
 
 /*