sh: Include <linux/io.h> in dac.h

Include <linux/io.h> to avoid depending on <linux/backlight.h>
for including it. Declares __raw_readb() and __raw_writeb().

Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202510282206.wI0HrqcK-lkp@intel.com/
Fixes: 243ce64b2b37 ("backlight: Do not include <linux/fb.h> in header file")
Cc: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Daniel Thompson (RISCstar) <danielt@kernel.org>
Cc: Simona Vetter <simona.vetter@ffwll.ch>
Cc: Lee Jones <lee@kernel.org>
Cc: Daniel Thompson <danielt@kernel.org>
Cc: Jingoo Han <jingoohan1@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>