The given maintainer script calls ldconfig. However, explicit calls in maintainer scripts should be replaced by a dpkg trigger.
Please replace the "ldconfig" call with an activate-noawait ldconfig trigger. With debhelper it is usually sufficient to simply add that line to debian/<package>.triggers.
If you use debhelper, this warning will appear if the package was compiled with debhelper before 9.20151004. Assuming all ldconfig invocations have been added by debhelper, this warning will disappear once the package is rebuilt with a newer version of debhelper.
Refer to https://lists.debian.org/debian-devel/2015/08/msg00412.html for details.