Fix KDE trying to install files always into the installation directory of polkitqt-1. The reason is

that POLKITQT-1_POLICY_FILES_INSTALL_DIR is a absolute filename in contrast to
POLKITQT-POLICY_FILES_INSTALL_DIR beeing a relative filename.

Fix it by replacing the POLKITQT-1_INSTALL_DIR part with CMAKE_INSTALL_PREFIX. The
FindPolkitQt-1.cmake file could need the same kind of solution i proposed for FindRuby.cmake.

CCMAIL: kde-buildsystem@kde.org
CCMAIL: drf54321@gmail.com