sched_ext: Use bitfields for boolean warning flags

Convert warned_zero_slice and warned_deprecated_rq in scx_sched struct to
single-bit bitfields. While this doesn't reduce struct size immediately,
it prepares for future bitfield additions.

v2: Update patch description.

Acked-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>