psi: Add a missing SPDX license header

Add the missing SPDX license header to
include/linux/psi.h
include/linux/psi_types.h
kernel/sched/psi.c

Signed-off-by: Liu Xinpeng <liuxp11@chinatelecom.cn>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/1635133586-84611-2-git-send-email-liuxp11@chinatelecom.cn