kernel/reboot.c: export pm_power_off_prepare

Export pm_power_off_prepare. It is needed to implement power off on
Freescale/NXP iMX6 based boards with external power management
integrated circuit (PMIC).
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>

kernel/reboot.c

@@ -49,6 +49,7 @@ int reboot_force;
  */
 
 void (*pm_power_off_prepare)(void);
+EXPORT_SYMBOL_GPL(pm_power_off_prepare);
 
 /**
  *	emergency_restart - reboot the system