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>