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.cpp

@@ -17,6 +17,7 @@
 #include <QDebug>
 #include <QSslCertificate>
 
+using namespace std;
 
 class SSLVerifyError : public std::runtime_error
 {