Install DefinitionDownloader header

Summary:
Makes it possible to use the exported definition downloader from an installed
syntax highlighting engine.

Reviewers: cullmann, vkrause

Reviewed By: vkrause

Subscribers: dhaumann, kwrite-devel, kde-frameworks-devel

Tags: Framework: Syntax Highlighting, Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D19215