simple_lmk: Add !PSI dependency

When PSI is enabled, lmkd in userspace will use PSI notifications to
perform low memory kills. Therefore, to ensure that Simple LMK is the
only active LMK implementation, add a !PSI dependency.

Signed-off-by: Sultan Alsawaf <sultan@kerneltoast.com>
Signed-off-by: Danny Lin <danny@kdrag0n.dev>