sched: Put task_group::idle under CONFIG_GROUP_SCHED_WEIGHT

sched: Put task_group::idle under CONFIG_GROUP_SCHED_WEIGHT

When build with CONFIG_GROUP_SCHED_WEIGHT && !CONFIG_FAIR_GROUP_SCHED,
the idle member is not defined:

kernel/sched/ext.c:3701:16: error: 'struct task_group' has no member named 'idle'
  3701 |         if (!tg->idle)
       |                ^~

Fix this by putting 'idle' under new CONFIG_GROUP_SCHED_WEIGHT.

tj: Move idle field upward to avoid breaking up CONFIG_FAIR_GROUP_SCHED block.

Fixes: e179e80c5d4f ("sched: Introduce CONFIG_GROUP_SCHED_WEIGHT")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202409220859.UiCAoFOW-lkp@intel.com/
Signed-off-by: Yu Liao <liaoyu15@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>