Fix missing include in Android build

Summary: Android builds are failing because QLabel is incomplete

Test Plan: The C++ Android build succeeds now, the APK creation is still failing with an unrelated issue

Reviewers: KStars, mutlaqja

Reviewed By: KStars, mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D22897