Fix OpenSUSE build error on incomplete type KStarsData.

Summary:
This fixes the error raised in https://build.kde.org/job/Extragear/job/kstars/job/stable-kf5-qt5%20SUSEQt5.12/382/console about the incomplete type KStarsData.
We have some deprecations on implicit copy constructors, but this differential does not touch that subject.

Test Plan: Attempt rebuild of stable-kf5-qt5%20SUSEQt5.12.

Reviewers: mutlaqja

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D27773