Use ${KXMLGUI_INSTALL_DIR}/lskat as destination for rc file

Fix the warning

> KXMLGUI file found at deprecated location ("/usr/share/lskat/lskatui.rc")
> -- please use ${KXMLGUI_INSTALL_DIR} to install this file instead.

by installing lskatui.rc at the corresponding location.

REVIEW:127868