backlight: Fix old-style function definition

Fix warning:

drivers/macintosh/via-pmu-backlight.c: In function ‘pmu_backlight_init’:
drivers/macintosh/via-pmu-backlight.c:140:13: warning: old-style function definition [-Wold-style-definition]
 void __init pmu_backlight_init()
             ^~~~~~~~~~~~~~~~~~
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

drivers/macintosh/via-pmu-backlight.c

@@ -137,7 +137,7 @@ void pmu_backlight_set_sleep(int sleep)
 }
 #endif /* CONFIG_PM */
 
-void __init pmu_backlight_init()
+void __init pmu_backlight_init(void)
 {
 	struct backlight_properties props;
 	struct backlight_device *bd;