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/security/validator.hpp b/src/security/validator.hpp
index 4ba4c43..1d74aa2 100644
--- a/src/security/validator.hpp
+++ b/src/security/validator.hpp
@@ -9,6 +9,8 @@
 #ifndef NDN_VALIDATOR_HPP
 #define NDN_VALIDATOR_HPP
 
+#include "../common.hpp"
+
 #include "../data.hpp"
 #include "../face.hpp"
 #include "public-key.hpp"