Address T4491 making KReportDpi a private non-exported class
Summary: Moved KReportDpi.h to KEReportDpi_p.h and associated changes
Test Plan: Ran autotests and kreportexample
Reviewers: staniek
Reviewed By: staniek
Tags: KReport
Maniphest Tasks: T4491
Differential Revision: https://phabricator.kde.org/D3800