actually install the version header

Summary:
we've generated it but never installed it :|
this enables lib consumers to check the kdnssd version without having to
pass around cmake data

CHANGELOG: now installs kdnssd_version.h to check the version of the lib

Test Plan: builds; install correctly

Reviewers: Frameworks, broulik

Reviewed By: broulik

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17068