Fix bug from missing header file

Summary: Include cmath header file, otherwise compilation depends on where this header file is included.

Test Plan: No specific test as I do not have actual example where issue is triggered.

Reviewers: tlatham, jback

Reviewed By: tlatham

Tags: #evtgen

Maniphest Tasks: T103

Differential Revision: https://phab.hepforge.org/D31