[1470] Reconcile declaration of Data

The .cc has it defined as "struct" and the .h file referenced a
"class".  This has now been harmonised as "struct".

src/lib/asiodns/udp_server.h

@@ -99,7 +99,7 @@ private:
      * This way the overhead of copying is lower, we copy only one shared
      * pointer instead of about 10 of them.
      */
-    class Data;
+    struct Data;
     boost::shared_ptr<Data> data_;
 };