kernel/sched.c: Suppress unused var warning

On UP:

 kernel/sched.c: In function 'wake_up_new_task':
 kernel/sched.c:2631: warning: unused variable 'cpu'

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>