debian/rules: Force building with system libsystemd-journal

Although auto-detection works, and prefers the system-wide installed
libsystemd-journal, do enforce its use, because optional does not make
sense on Debian.

This is a guard, so that this functionality does not get accidentally
left out in the future.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>