security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/public-key.hpp b/src/security/public-key.hpp
index 8fc5dab..a332f02 100644
--- a/src/security/public-key.hpp
+++ b/src/security/public-key.hpp
@@ -6,8 +6,8 @@
  * See COPYING for copyright and distribution information.
  */
 
-#ifndef NDN_PUBLIC_KEY_HPP
-#define NDN_PUBLIC_KEY_HPP
+#ifndef NDN_SECURITY_PUBLIC_KEY_HPP
+#define NDN_SECURITY_PUBLIC_KEY_HPP
 
 #include "../common.hpp"
 
@@ -80,6 +80,6 @@
 std::ostream &
 operator <<(std::ostream &os, const PublicKey &key);
 
-}
+} // namespace ndn
 
-#endif
+#endif //NDN_SECURITY_PUBLIC_KEY_HPP