Fix compilation with boost 1.66.0

Change-Id: Ibf2883a974c834a5281c39a551e6d467d4454501
diff --git a/src/transport/unix-transport.hpp b/src/transport/unix-transport.hpp
index d54f02b..d337252 100644
--- a/src/transport/unix-transport.hpp
+++ b/src/transport/unix-transport.hpp
@@ -25,13 +25,7 @@
 #include "transport.hpp"
 #include "../util/config-file.hpp"
 
-namespace boost {
-namespace asio {
-namespace local {
-class stream_protocol;
-} // namespace local
-} // namespace asio
-} // namespace boost
+#include <boost/asio/local/stream_protocol.hpp>
 
 namespace ndn {