ECMGeneratePriFile: fix for ECM_MKSPECS_INSTALL_DIR being absolute
Test Plan: works as before for the case where it's relative.
Reviewers: cgiboudeaux, vatra, kfunk, apol, vkrause
Reviewed By: cgiboudeaux
Subscribers: kde-frameworks-devel, kde-buildsystem
Tags: Frameworks, Build System
Differential Revision: https://phabricator.kde.org/D29524