Fix compile error without c-ares

src/DHTEntryPointNameResolveCommand.cc

@@ -61,7 +61,9 @@ DHTEntryPointNameResolveCommand::DHTEntryPointNameResolveCommand
  const std::vector<std::pair<std::string, uint16_t> >& entryPoints):
   Command(cuid),
   e_(e),
+#ifdef ENABLE_ASYNC_DNS
   asyncNameResolverMan_(new AsyncNameResolverMan()),
+#endif // ENABLE_ASYNC_DNS
   entryPoints_(entryPoints.begin(), entryPoints.end()),
   numSuccess_(0),
   bootstrapEnabled_(false)

src/NameResolveCommand.cc

@@ -57,7 +57,9 @@ NameResolveCommand::NameResolveCommand
  const SharedHandle<UDPTrackerRequest>& req)
   : Command(cuid),
     e_(e),
+#ifdef ENABLE_ASYNC_DNS
     asyncNameResolverMan_(new AsyncNameResolverMan()),
+#endif // ENABLE_ASYNC_DNS
     req_(req)
 {
 #ifdef ENABLE_ASYNC_DNS