kernel: move volatile from k_thread.prio to k_thread.sched_locked

When prio and sched_locked were moved into a struct together to create a
union with the combined preempt field, the volatile qualifier moved from
sched_locked to prio by mistake.

Change-Id: I5a8e01324f14e77e3d7162c12515471826023633
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>