defined UDPQuery::~Callback explictly, making it virutal in order to avoid getting the "non-virtual destructor" warning from a certain version of gcc.

It caused buildbot failures, and I confirmed the fix on sol-10.lab, so committing it to trunk directly without having an explict review.
The fix should be quite trivial.


git-svn-id: svn://bind10.isc.org/svn/bind10/trunk@3905 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/asiolink/internal/udpdns.h

@@ -251,9 +251,11 @@ public:
     };
     /// Abstract callback for the UDPQuery.
     class Callback {
-        public:
-            /// This will be called when the UDPQuery is completed
-            virtual void operator()(Result result) = 0;
+    public:
+        virtual ~Callback() {}
+
+        /// This will be called when the UDPQuery is completed
+        virtual void operator()(Result result) = 0;
     };
     ///
     /// \brief Constructor.