add virtual destructor

src/wifi/helper/wifi-helper.cc

@@ -52,6 +52,10 @@ WifiMacHelper::~WifiMacHelper ()
 {
 }
 
+WifiHelper::~WifiHelper ()
+{
+}
+
 WifiHelper::WifiHelper ()
   : m_standard (WIFI_PHY_STANDARD_80211a)
 {

src/wifi/helper/wifi-helper.h

@@ -88,6 +88,8 @@ public:
 class WifiHelper
 {
 public:
+  virtual ~WifiHelper ();
+
   /**
    * Create a Wifi helper in an empty state: all its parameters
    * must be set before calling ns3::WifiHelper::Install