sched: Move Kconfig.preempt to sched/Kconfig

Kconfig.preempt already contains more than just preemption configs (see
CONFIG_SCHED_CORE), and a subsequent patch will introduce more
scheduler-specific configs.

Move the file to the scheduler directory.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>