Use KDE_INSTALL_TARGETS_DEFAULT_ARGS when installing targets

Summary:
Otherwise the targets' INTERFACE_INCLUDE_DIRECTORIES will contain
$PREFIX/include/KF5 instead of $PREFIX/include. This causes build
failures on the CI because libkgapi no longer installs the include files
under the KF5 prefix. Thus cmake fails with errors such as:

"Imported target KPim::GAPICore includes non-existent path"

Reviewers: dvratil

Differential Revision: https://phabricator.kde.org/D5015