security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/certificate.cpp b/src/security/certificate.cpp
index 3778a80..f0dfdf2 100644
--- a/src/security/certificate.cpp
+++ b/src/security/certificate.cpp
@@ -265,4 +265,4 @@
   // keyRoot->accept(printVisitor, string(""));
 }
 
-}
+} // namespace ndn