drm/nouveau/led: explicitly include linux/leds.h

Instead of relying on an accidental, transitive inclusion of linux/leds.h
use it directly.

Also drop the forware definition of struct led_classdev that is now
provided by linux/leds.h.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230215-power_supply-leds-nouveau-v1-1-ea93bfa0ba7e@weissschuh.net