Use INTERFACE IMPORTED target type instead of ALIAS for compatibility targets

Summary: Apparently IMPORTED_GLOBAL is a 3.11 feature and Neon uses 3.10

Test Plan: Use CMake 3.10, plasma-workspace compiles again.

Reviewers: #plasma, ngraham, kmaterka

Reviewed By: ngraham, kmaterka

Subscribers: kmaterka, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D29529