debian: Update for syslog-ng 3.5.6

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
debian: Use dh_installinit --restart-after-upgrade

Instead of hardcoding essentially the same thing (badly), use
dh_installinit --restart-after-upgrade.

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