security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/validator-null.hpp b/src/security/validator-null.hpp
index d1d8faa..eb186c6 100644
--- a/src/security/validator-null.hpp
+++ b/src/security/validator-null.hpp
@@ -6,8 +6,8 @@
  * See COPYING for copyright and distribution information.
  */
 
-#ifndef NDN_VALIDATOR_NULL_HPP
-#define NDN_VALIDATOR_NULL_HPP
+#ifndef NDN_SECURITY_VALIDATOR_NULL_HPP
+#define NDN_SECURITY_VALIDATOR_NULL_HPP
 
 #include "validator.hpp"
 
@@ -37,6 +37,6 @@
   { onValidated(interest.shared_from_this()); }
 };
 
-}
+} // namespace ndn
 
-#endif
+#endif //NDN_SECURITY_VALIDATOR_NULL_HPP