[1603] cleanup: constify a member function.

src/lib/dns/messagerenderer.cc

@@ -178,7 +178,7 @@ struct MessageRenderer::MessageRendererImpl {
     }
 
     uint16_t findOffset(const OutputBuffer& buffer, InputBuffer& name_buf,
-                        size_t hash, bool case_sensitive)
+                        size_t hash, bool case_sensitive) const
     {
         // Find a matching entry, if any.  We use some heuristics here: often
         // the same name appers consecutively (like repeating the same owner