sched: Mark boot-cpu active before smp_init()

A UP machine has 1 active cpu, not having the boot-cpu in the
active map when starting the scheduler confuses things.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.423469527@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>