backlight/corgi-lcd: Include <linux/backlight.h>

Resolves the proxy include via <linux/fb.h>, which does not require the
backlight header.

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Acked-by: Lee Jones <lee@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20240219093941.3684-2-tzimmermann@suse.de