powerpc: Fix old-style function definition

Fix warnings such as:

arch/powerpc/platforms/powermac/backlight.c: In function ‘pmac_backlight_get_legacy_brightness’:
arch/powerpc/platforms/powermac/backlight.c:189:5: error: old-style function definition [-Werror=old-style-definition]
 int pmac_backlight_get_legacy_brightness()
     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powermac/backlight.c

@@ -186,7 +186,7 @@ int pmac_backlight_set_legacy_brightness(int brightness)
 	return __pmac_backlight_set_legacy_brightness(brightness);
 }
 
-int pmac_backlight_get_legacy_brightness()
+int pmac_backlight_get_legacy_brightness(void)
 {
 	int result = -ENXIO;
 
@@ -205,12 +205,12 @@ int pmac_backlight_get_legacy_brightness()
 	return result;
 }
 
-void pmac_backlight_disable()
+void pmac_backlight_disable(void)
 {
 	atomic_inc(&kernel_backlight_disabled);
 }
 
-void pmac_backlight_enable()
+void pmac_backlight_enable(void)
 {
 	atomic_dec(&kernel_backlight_disabled);
 }