security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/validator.cpp b/src/security/validator.cpp
index 6eec2d0..d0c808b 100644
--- a/src/security/validator.cpp
+++ b/src/security/validator.cpp
@@ -256,4 +256,4 @@
   return result;
 }
 
-}
+} // namespace ndn