avoid compilation warning about undefined MobilityModel class.

src/helper/mobility-helper.cc

@@ -35,6 +35,8 @@ MobilityHelper::MobilityHelper ()
 								   "Y", ConstantVariable (0.0));
   m_mobility.SetTypeId ("ns3::StaticMobilityModel");
 }
+MobilityHelper::~MobilityHelper ()
+{}
 void 
 MobilityHelper::EnableNotifier (void)
 {

src/helper/mobility-helper.h

@@ -40,6 +40,7 @@ class MobilityHelper
 {
 public:
   MobilityHelper ();
+  ~MobilityHelper ();
 
   /**
    * After this method is called, every call to MobilityHelper::Layout