Fix missing include

Odd that it only started causing problems for Travis after making a
couple of commits that didn't change any C++ code...

src/backend/game/game.cpp

@@ -28,6 +28,7 @@
 #include "../error.h"
 
 #include <thread>
+#include <cmath>
 
 #include <boost/algorithm/string.hpp>
 #include <boost/locale.hpp>