Bring FindUDev.cmake up to ECM standards

Summary:
This prepares it to be upstreamed to ECM, as it's also needed by KWin
and qtbase.

Reviewers: Build System, cgiboudeaux, apol

Reviewed By: cgiboudeaux, apol

Subscribers: aacid, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D18947