use std::sqrt in vcldemo.cxx

Change-Id: I24d8ba15ee267d0cad3b063df9b7cfd8d284f4ee
Reviewed-on: https://gerrit.libreoffice.org/18591Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>