Fix typo in name of installed CMake Config: Deskop->Desktop
Summary: Also no longer create unused file qqc2desktopstyle_version.h
Reviewers: mart, broulik, davidedmundson, apol
Reviewed By: davidedmundson, apol
Subscribers: apol, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D23005