sched: fix build error, provide partition_sched_domains() unconditionally

provide an empty partition_sched_domains() definition for the UP case:

 include/linux/cpuset.h: In function ‘rebuild_sched_domains':
 include/linux/cpuset.h:163: error: implicit declaration of function ‘partition_sched_domains'

Signed-off-by: Ingo Molnar <mingo@elte.hu>