security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/sec-tpm.cpp b/src/security/sec-tpm.cpp
index 13b04a8..a52d839 100644
--- a/src/security/sec-tpm.cpp
+++ b/src/security/sec-tpm.cpp
@@ -284,4 +284,4 @@
 }
 
 
-}//ndn
+} // namespace ndn