Fix compilation with latest ndn-cxx

Change-Id: Iaf468a75466134d18ecbb1fef66e4cb401d781a4
diff --git a/src/leaf-container.hpp b/src/leaf-container.hpp
index e1d30bd..f08f8f9 100644
--- a/src/leaf-container.hpp
+++ b/src/leaf-container.hpp
@@ -44,7 +44,7 @@
   std::size_t
   operator()(const Name& prefix) const
   {
-    ndn::ConstBufferPtr buffer =
+    ConstBufferPtr buffer =
       ndn::util::Sha256::computeDigest(prefix.wireEncode().wire(), prefix.wireEncode().size());
 
     BOOST_ASSERT(buffer->size() > sizeof(std::size_t));