sched/fair: Only kick nohz balance when runqueue has more than 1 task

Misfit task will go through active migration path so there is no need
to check in the nohz balance case.

Change-Id: Ic05035d8c745797c4a5cbe52b7cdbd98edb352f3
Signed-off-by: Runmin Wang <runminw@codeaurora.org>