Update sio_client_impl.cpp

Use `std::` when required. 

Even though there is a `using namespace std` this should be removed and we should add `std::` whenever possible

src/internal/sio_client_impl.cpp

@@ -563,10 +563,10 @@ namespace sio
                 if (message && message->get_flag() == message::flag_object)
                 {
                     const object_message* obj_ptr = static_cast<object_message*>(message.get());
-                    const map<string, message::ptr>* values = &(obj_ptr->get_map());
+                    const std::map<std::string, message::ptr>* values = &(obj_ptr->get_map());
                     auto it = values->find("sid");
                     if (it != values->end()) {
-                        m_sid = static_pointer_cast<string_message>(it->second)->get_string();
+                        m_sid = std::static_pointer_cast<string_message>(it->second)->get_string();
                     }
                 }
             }