Fix the std::string error introduced in integration.

python/google/protobuf/pyext/message.h

@@ -54,7 +54,7 @@ class MessageFactory;
 
 #ifdef _SHARED_PTR_H
 using shared_ptr;
-using std::std::string;
+using ::std::string;
 #else
 using internal::shared_ptr;
 #endif