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>
diff --git a/arch/um/Kconfig.common b/arch/um/Kconfig.common
index fd44385..347bcd4 100644
--- a/arch/um/Kconfig.common
+++ b/arch/um/Kconfig.common
@@ -62,3 +62,8 @@
 config SUBARCH
 	string
 	option env="SUBARCH"
+
+config NR_CPUS
+	int
+	range 1 1
+	default 1