sched_ext: Minor cleanups in kernel/sched/ext.h

- scx_ops_cpu_preempt is only used in kernel/sched/ext.c and doesn't need to
  be global. Make it static.

- Relocate task_on_scx() so that the inline functions are located together.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: David Vernet <void@manifault.com>