Fix a compiler warning: not defining a virtual destructor results in undefined behavior

simgear/emesary/IReceiver.hxx

@@ -31,6 +31,8 @@ namespace simgear
         class IReceiver : public SGReferenced
         {
         public:
+            virtual ~IReceiver() = default;
+
             /// Receive notification - must be implemented
             virtual ReceiptStatus Receive(INotification& message) = 0;
 
@@ -47,4 +49,4 @@ namespace simgear
 
     }
 }
-#endif
+#endif