all: avoid usage of deprecated namespace ndn::Tlv

refs #2079

Change-Id: I3d8748929706726b55fe1a9a0f6ae00857ea3547
diff --git a/common.hpp b/common.hpp
index 3b9d2ce..cf236ca 100644
--- a/common.hpp
+++ b/common.hpp
@@ -84,8 +84,9 @@
 using ndn::Block;
 
 namespace tlv {
-using namespace ndn::Tlv;
-}
+// Don't write "namespace tlv = ndn::tlv", because NFD can add other members into this namespace.
+using namespace ndn::tlv;
+} // namespace tlv
 
 namespace name = ndn::name;
 namespace time = ndn::time;