um: Set number of CPUs

Define NR_CPUS required by the timer subsystem.

Fixes this make warning:

    scripts/kconfig/conf  --oldconfig arch/x86/um/Kconfig
    kernel/time/Kconfig:155:warning: range is invalid

Signed-off-by: Nikola Kotur <kotnick@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>