Fix cmake warning: Migrate ECMPackageConfigHelpers -> CMakePackageConfigHelpers

Test Plan: still compiles; kblocks still compiles after installing libkdegames from sources

Reviewers: #kde_games, aacid

Reviewed By: aacid

Subscribers: kde-games-devel, mlaurent

Differential Revision: https://phabricator.kde.org/D21677