Don't export internal helper executables

Summary:
Exporting the executables will make the cmake config file check for their
presence when used, which isn't really necessary, and is actually a problem
for cross-compilation, where the helper executables don't exist for the
target but for the host.

Reviewers: Frameworks, cordlandwehr, apol

Reviewed By: apol

Subscribers: apol, sitter, sandsmark, habacker, dfaure

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7008