All Fujitsu-Siemens Lifebook systems are x86-based, so we might
as well make MOUSE_PS2_LIFEBOOK depend on X86. This will avoid
surprising things like:

arch/arm/configs/s3c2410_defconfig:CONFIG_MOUSE_PS2_LIFEBOOK=y
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>