HID: HID_APPLETB_BL should depend on X86

The Apple Touch Bar is only present on x86 MacBook Pros.  Hence add a
dependency on X86, to prevent asking the user about this driver when
configuring a kernel for a different architecture.

Fixes: 1fd41e5e ("HID: hid-appletb-bl: add driver for the backlight of Apple Touch Bars")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>