Summary:
Out-of-source builds would fail, as well as builds with a recent GCC.

However, this is more of an emergency build fix. Including the moc file
instead of fixing the linking error is still a hack and should
be fixed properly (see D7305).

Test Plan:
- Builds on KDE Neon (GCC 5.4) as well as openSUSE Tumbleweed (GCC 7.1).
- Out-of-source and in-source builds succeed.

Reviewers: cfeck, ngraham, ckertesz

Reviewed By: ngraham

Differential Revision: https://phabricator.kde.org/D8001