FROMLIST: 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>

(not upstream yet, latest version published at: https://lore.kernel.org/patchwork/patch/1052415/

)

Bug: 127712811
Bug: 129157727
Test: lmkd in PSI mode
Change-Id: I249c6d2271f93a7975f1622faf2d2b4196b701bc
Signed-off-by: Suren Baghdasaryan <surenb@google.com>