refactor header includes to allow kthread.h inclusion in psi_types.h

kthread.h can't be included in psi_types.h because it creates a circular
inclusion with kthread.h eventually including psi_types.h and complaining
on kthread structures not being defined because they are defined further
in the kthread.h. Resolve this by removing psi_types.h inclusion from the
headers included from kthread.h.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Change-Id: I44e2f6fce8f1a3e9e6a8c1934d7a4a047917ec3f
Patch-mainline: linux-mm @ 03/19/2019, 23:56
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>