Fix warning treated as error on bionic.

Summary:
This is a hotfix for https://build.neon.kde.org/job/bionic_unstable_extras_kstars_bin_amd64/446/console.
This build treats the warning "format not a string literal and no format arguments" as an error.

Test Plan: Let Jenkins rebuild on that platform.

Reviewers: KStars, mutlaqja

Reviewed By: KStars, mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D27820