debian: Move the systemd-journal() source to its own package

To be able to build syslog-ng on non-linux architectures, move the
systemd-journal() source to its own package. This way we do not have to
do gross hacks in debian/syslog-ng-core.install. Instead, syslog-ng-core
depends on this new syslog-ng-mod-journal package on linux-any (which is
a linux-any package itself).

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