Don't export kded5 as a CMake target

Summary:
kded5 is not needed during the build, but exporting it will make the
corresponding find_package call verify its presence nevertheless. This
is inconvenient when cross-compiling, where kded5 typically does not
exist for the host system.

Reviewers: Frameworks, apol

Reviewed By: apol

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D9595