sched/fair: Mark tg_is_idle() an inline in the !CONFIG_FAIR_GROUP_SCHED case

sched/fair: Mark tg_is_idle() an inline in the !CONFIG_FAIR_GROUP_SCHED case

It's not actually used in the !CONFIG_FAIR_GROUP_SCHED case:

  kernel/sched/fair.c:488:12: warning: ‘tg_is_idle’ defined but not used [-Wunused-function]

Keep around a placeholder nevertheless, for API completeness. Mark it inline,
so the compiler doesn't think it must be used.

Fixes: 84f52be93429: ("sched: Cgroup SCHED_IDLE support")
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Josh Don <joshdon@google.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Vincent Guittot <vincent.guittot@linaro.org>