package/systemd: bump to v243.4

Upstream systemd-stable has started tagging point releses.

The commit we currently used has now been tagged as v243.3, and this
brings us to v243.4.

Signed-off-by: Jérémy Rosen <jeremy.rosen@smile.fr>
[yann.morin.1998@free.fr:
  - expand commit log to explain previous version
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>