mfd: ab8500-sysctrl: Fix sparse warning

Fix sparse warning:

drivers/mfd/ab8500-sysctrl.c:26:6: warning: symbol 'ab8500_power_off' was not declared. Should it be static?
Acked-by: NLinus Walleij <linus.walleij@linaro.org>
Signed-off-by: NFabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

drivers/mfd/ab8500-sysctrl.c

@@ -23,7 +23,7 @@
 
 static struct device *sysctrl_dev;
 
-void ab8500_power_off(void)
+static void ab8500_power_off(void)
 {
 	sigset_t old;
 	sigset_t all;