strongswam: disable systemd support in meta-oe and reenable it in meta-systemd

* fixes:
strongswan-5.0.0: strongswan: Files/directories were installed but not shipped
  /lib
  /lib/systemd
  /lib/systemd/system
  /lib/systemd/system/strongswan.service

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>