Fixing usages of ptr_lib::shared_ptr

In certain cases, importing both std:: and ndn::ptr_lib namespaces
causes ambiguity for shared_ptr type.

Change-Id: Iddc02353e1615b2387277c56185a1dd873d54459
diff --git a/include/ndn-cpp/data.hpp b/include/ndn-cpp/data.hpp
index 43615d3..f0de52f 100644
--- a/include/ndn-cpp/data.hpp
+++ b/include/ndn-cpp/data.hpp
@@ -313,10 +313,10 @@
   void 
   onChanged();
   
-  ptr_lib::shared_ptr<Signature> signature_;
   Name name_;
   MetaInfo metaInfo_;
   Blob content_;
+  ptr_lib::shared_ptr<Signature> signature_;
   SignedBlob defaultWireEncoding_;
 };