debian/rules: Generate correct shlibs file for syslog-ng-core

The logic of when to generate the shlibs file was reversed, and as such,
when building syslog-ng-core, we didn't pass the appropriate parameters.
This patch fixes the logic, and syslog-ng-core will now have correct
shlibs, with versioned depends.

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