openldap-2.6: Add tmpfiles.d snippet

[cjwatson]

This ensures that the appropriate subdirectory of /run exists when running in a VM, where /run is a tmpfs.

Related: https://docs.google.com/document/d/1pydotE6GRdgWP9xmO9YFFX6_f7PdMa7wMWRQSzH8bFw