Fix the build which is broken on Macs by adding include for cmath

Summary: Previous submit needs an include of <cmath> to compile on Macs.

Test Plan: Compile kstars on a mac, won't work without this.

Reviewers: mutlaqja, TallFurryMan

Reviewed By: mutlaqja

Subscribers: kde-edu

Tags: KDE Edu

Differential Revision: https://phabricator.kde.org/D28716