[kde-frameworks/kxmlgui] Add attica USE flag.

Package-Manager: portage-2.2.7
[eclass] Only set S when building a release.

When building a live version, the git eclass automatically
unpacks it into the default location.
[kde-frameworks/attica] Correct REQUIRED_USE operator.

attica can now be built against both Qt4 and Qt5 in parallel.

Package-Manager: portage-2.2.7