encoding: add missing include in nfd-constants.hpp

refs #2091

Change-Id: I0319506eac91b5a97a0a882dc95473a4bb56eb0a
diff --git a/src/encoding/nfd-constants.hpp b/src/encoding/nfd-constants.hpp
index 54794bb..6b00c42 100644
--- a/src/encoding/nfd-constants.hpp
+++ b/src/encoding/nfd-constants.hpp
@@ -22,6 +22,8 @@
 #ifndef NDN_ENCODING_NFD_CONSTANTS_HPP
 #define NDN_ENCODING_NFD_CONSTANTS_HPP
 
+#include "../common.hpp"
+
 namespace ndn {
 namespace nfd {