Use correct INSTALL_TARGETS_DEFAULT_ARGS

Non-KF targets must use KDE_INSTALL_TARGETS_DEFAULT_ARGS. The KF_
variables adds /include/KF[56] to
INTERFACE_INCLUDE_DIRECTORIES and that causes cmake errors for users
of this library if /include/KF[56] doesn't exist
(e.g. because one uses KF libraries provided by the distro).