layer.conf: add systemd-serialgetty to SIGGEN_EXCLUDERECIPES_ABISAFE

* it was imported from meta-systemd without SIGGEN_EXCLUDERECIPES_ABISAFE
  change

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>