sched: Fix update_group_power() prototype placement to fix build warning when !CONFIG_SMP

All warnings:

   In file included from kernel/sched/core.c:85:0:
   kernel/sched/sched.h:1036:39: warning: 'struct sched_domain' declared inside parameter list
   kernel/sched/sched.h:1036:39: warning: its scope is only this definition or declaration, which is probably not what you want

It's because struct sched_domain is defined inside #if CONFIG_SMP,
while update_group_power() is declared unconditionally.

Fix this warning by declaring update_group_power() only if
CONFIG_SMP=n.

Build tested with CONFIG_SMP enabled and then disabled.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/5137F4BA.2060101@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>