security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/certificate.hpp b/src/security/certificate.hpp
index 260638a..35934d0 100644
--- a/src/security/certificate.hpp
+++ b/src/security/certificate.hpp
@@ -6,8 +6,8 @@
  * See COPYING for copyright and distribution information.
  */
 
-#ifndef NDN_CERTIFICATE_HPP
-#define NDN_CERTIFICATE_HPP
+#ifndef NDN_SECURITY_CERTIFICATE_HPP
+#define NDN_SECURITY_CERTIFICATE_HPP
 
 #include "../common.hpp"
 #include "../data.hpp"
@@ -153,4 +153,4 @@
 
 } // namespace ndn
 
-#endif
+#endif //NDN_SECURITY_CERTIFICATE_HPP