Remove using namespace std from header file

It's considered bad form to import things into the global namespace in a
header. Put it in the cpp files where it is needed instead.

src/qt/paymentrequestplus.h

@@ -24,7 +24,7 @@ public:
     PaymentRequestPlus() { }
 
     bool parse(const QByteArray& data);
-    bool SerializeToString(string* output) const;
+    bool SerializeToString(std::string* output) const;
 
     bool IsInitialized() const;
     QString getPKIType() const;