Fix build errors if PREFIX is different from ECM's PREFIX.
Fixes this issue:
When plasma-framework and extra-cmake-modules reside in different prefixes, building packages depending on PlasmaQuick fails with
File or directory
/gnu/store/…-extra-cmake-modules-5.70.0/include/KF5
referenced by variable PlasmaQuick_INCLUDE_DIR does not exist !
"/gnu/store/…-extra-cmake-modules-5.70.0" is the of PREFIX of ECM - and obviously PlasmaQuick_INCLUDE_DIR should not point there but to the PREFIX of plasma-framework.
See https://bugs.kde.org/424483 for details.
The former way of including this file is only used twice all over KDE (see search hat Github https://github.com/search?p=2&q=org%3AKDE+KDEInstallDirs+extension%3Acmake&type=Code): here and in kpackage.