Declare StateVariableBase destructor as virtual so that the correct d…

…estructor is called for derived classes. Coverity defect 701004

mythtv/libs/libmythupnp/eventing.h

@@ -107,6 +107,7 @@ class UPNP_PUBLIC  StateVariableBase
             m_sName   = sName;
             gettimeofday( (&m_ttLastChanged), NULL );
         }
+        virtual ~StateVariableBase() {};
 
         virtual QString ToString() = 0;
 };