build: Experimental support to build using precompiled headers

This type of build needs to be explicitly enabled using --with-pch
configure option.

Change-Id: I02dc304187d54c3cf40f1d51b1b11b21f868e225
diff --git a/src/transport/unix-transport.hpp b/src/transport/unix-transport.hpp
index c2ee204..26621d7 100644
--- a/src/transport/unix-transport.hpp
+++ b/src/transport/unix-transport.hpp
@@ -8,7 +8,7 @@
 #ifndef NDN_UDPTRANSPORT_HPP
 #define NDN_UDPTRANSPORT_HPP
 
-#include <string>
+#include "../common.hpp"
 #include "transport.hpp"
 
 namespace ndn {