[Qt] remove std namespace polution from code

src/qt/paymentrequestplus.cpp

@@ -19,8 +19,6 @@
 #include <QDebug>
 #include <QSslCertificate>
 
-using namespace std;
-
 class SSLVerifyError : public std::runtime_error
 {
 public:
@@ -49,7 +47,7 @@ bool PaymentRequestPlus::parse(const QByteArray& data)
     return true;
 }
 
-bool PaymentRequestPlus::SerializeToString(string* output) const
+bool PaymentRequestPlus::SerializeToString(std::string* output) const
 {
     return paymentRequest.SerializeToString(output);
 }