sched/core: Make some functions static

Fix these sparse warnings:

  kernel/sched/core.c:6577:11: warning: symbol 'min_cfs_quota_period' was not declared. Should it be static?
  kernel/sched/core.c:6657:5: warning: symbol 'tg_set_cfs_quota' was not declared. Should it be static?
  kernel/sched/core.c:6670:6: warning: symbol 'tg_get_cfs_quota' was not declared. Should it be static?
  kernel/sched/core.c:6683:5: warning: symbol 'tg_set_cfs_period' was not declared. Should it be static?
  kernel/sched/core.c:6693:6: warning: symbol 'tg_get_cfs_period' was not declared. Should it be static?
  kernel/sched/fair.c:2596:6: warning: symbol 'task_tick_numa' was not declared. Should it be static?

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20190418144713.34332-1-yuehaibing@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>