debian/rules: Fixed a typo in a comment.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
debian/rules: Add looser shlibs dependency on syslog-ng-core

When setting the shlibs dependency info for syslog-ng-core (the
dependency that should be used by modules built against syslog-ng),
loosen the relation a bit. Instead of being a strictly equal relation,
depend on >= $current_version and << $current_version+1.

We can't have a looser relation, because modules are installed into
/usr/lib/syslog-ng/$VERSION.

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