debian: Install a custom scl.conf

Upstream uses an scl.conf that includes the various snippets one-by-one,
explicitly. This does not allow us to split them out of syslog-ng-core
into various other packages where they may be more fitting. To fix this,
ship our own scl.conf, that uses a wildcard include.

(See balabit/syslog-ng#296 for the upstream pull request.)

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