acpi_video: kernel build error if !INPUT

Commit e9dab1960ac9746fa34eff726b81635147615a79
(ACPI video hotkey: export missing ACPI video hotkey events via input layer)
exports ACPI video hotkey events via input layer. But this breaks kernel
build if ACPI_VIDEO && !INPUT:

 LD      .tmp_vmlinux1
drivers/built-in.o: In function `acpi_video_bus_remove':
drivers/acpi/video.c:2007: undefined reference to `input_unregister_device'
    ...

Signed-off-by: Andreas Herrmann <aherrman@arcor.de>
Signed-off-by: Len Brown <len.brown@intel.com>