Memoize::SDBM_File(3p) on OpenBSD

The text bcbebd5af5b7, as OpenBSD 7.2, OpenBSD 7.1, OpenBSD 7.0, OpenBSD 6.9 shipped it.

glue to provide EXISTS for SDBM_File for Storable use

Memoize::SDBM_File(3p) on OpenBSD now