MagFieldUtils: Fix compilation with gcc10.

Need <cmath> for std::isnan.

MagneticField/MagFieldUtils/src/MagFieldTestbedAlg.cxx

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 */
 
 ///////////////////////////////////////////////////////////////////
@@ -39,12 +39,15 @@
 // performance test
 #include <time.h>
 #include <vector>
+#include <cmath> // isnan
 
 namespace {
 const double solenoidRadius = 1075.;
 const double solenoidZhalf = 2820.;
 }
 
+using std::isnan;
+
 ///////////////////////////////////////////////////////////////////
 // Public methods:
 ///////////////////////////////////////////////////////////////////