drm/panel-ili9341: Include <linux/backlight.h>

Include <linux/backlight.h> for devm_of_find_backlight().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221103151446.2638-10-tzimmermann@suse.de