Fix compilation against the latest ndn-cxx library

Change-Id: Ib9ae9c1601359a81a7f131ab63803f11adba7f38
diff --git a/core/leaf.cpp b/core/leaf.cpp
index a35d489..36dd275 100644
--- a/core/leaf.cpp
+++ b/core/leaf.cpp
@@ -166,9 +166,9 @@
 {
   size_t totalLength = 0;
 
-  totalLength += ndn::prependNonNegativeIntegerBlock(block, tlv::SignerSeqNo, m_signerSeqNo);
-  totalLength += ndn::prependNonNegativeIntegerBlock(block, tlv::DataSeqNo, m_dataSeqNo);
-  totalLength += ndn::prependNonNegativeIntegerBlock(block, tlv::Timestamp, m_timestamp);
+  totalLength += prependNonNegativeIntegerBlock(block, tlv::SignerSeqNo, m_signerSeqNo);
+  totalLength += prependNonNegativeIntegerBlock(block, tlv::DataSeqNo, m_dataSeqNo);
+  totalLength += prependNonNegativeIntegerBlock(block, tlv::Timestamp, m_timestamp);
   totalLength += m_dataName.wireEncode(block);
 
   totalLength += block.prependVarNumber(totalLength);