[1470] Make io_fetch.cc and .h consistent about type of IOFetchData

Was declared as "class" in one and "struct" in another - now declared
as "struct" in both.

src/lib/asiodns/io_fetch.h

@@ -35,7 +35,7 @@ namespace isc {
 namespace asiodns {
 
 // Forward declarations
-class IOFetchData;
+struct IOFetchData;
 
 /// \brief Upstream Fetch Processing
 ///