Fix CMakeLists.txt issues

Summary: Remove redundant install(FILES) and fix PREFIX for KPeople/Widgets (so pretty CamelCase headers would work)

Test Plan: It compiles, it works.

Reviewers: Frameworks, dvratil, apol

Reviewed By: dvratil

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15497