psi: make psi_enable static

psi_enable is not used outside of psi.c, make it static.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Change-Id: I37cef0577f465834725f6c080732d11c5486efd0
Patch-mainline: linux-mm @ 03/19/2019, 23:56
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>