security: Add doxygen comments

Change-Id: Ieafe4d13513e626d84ef9e621694f69844785018
diff --git a/src/security/identity-certificate.cpp b/src/security/identity-certificate.cpp
index 44d6588..5fddbfc 100644
--- a/src/security/identity-certificate.cpp
+++ b/src/security/identity-certificate.cpp
@@ -90,4 +90,4 @@
   return tmpName.getSubName(0, i).append(tmpName.getSubName(i + 1, tmpName.size() - i - 1));
 }
 
-}
+} // namespace ndn