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/src/encoding/der/visitor/public-key-visitor.cpp b/src/encoding/der/visitor/public-key-visitor.cpp
index 87e4e6c..62a4b7f 100644
--- a/src/encoding/der/visitor/public-key-visitor.cpp
+++ b/src/encoding/der/visitor/public-key-visitor.cpp
@@ -12,7 +12,6 @@
 #include "public-key-visitor.hpp"
 
 using namespace std;
-using namespace ndn::ptr_lib;
 
 namespace ndn {
 
@@ -24,10 +23,10 @@
   const DerNodePtrList& children = derSeq.getChildren();
 
   SimpleVisitor simpleVisitor;
-  shared_ptr<DerSequence> algoSeq = dynamic_pointer_cast<DerSequence>(children[0]); 
+  ptr_lib::shared_ptr<DerSequence> algoSeq = dynamic_pointer_cast<DerSequence>(children[0]); 
   OID algorithm = ndnboost::any_cast<OID>(algoSeq->getChildren()[0]->accept(simpleVisitor));  
   Blob raw = derSeq.getRaw();   
-  return ndnboost::any(shared_ptr<PublicKey>(new PublicKey(algorithm, raw)));    
+  return ndnboost::any(ptr_lib::shared_ptr<PublicKey>(new PublicKey(algorithm, raw)));    
 }
 
 } // der