Include cmath header to get std::pow

--- gringo-4.5.2-source.orig/libgringo/src/term.cc
+++ gringo-4.5.2-source/libgringo/src/term.cc
@@ -21,6 +21,7 @@
 #include "gringo/term.hh"
 #include "gringo/logger.hh"
 #include "gringo/graph.hh"
+#include <cmath>
 
 namespace Gringo {