qt: define QT_NO_KEYWORDS

QT_NO_KEYWORDS prevents Qt from defining the `foreach`, `signals`,
`slots` and `emit` macros.

Avoid overlap between Qt macros and boost - for example #undef hackiness
in #6421.

src/qt/receiverequestdialog.h

@@ -32,7 +32,7 @@ public:
     explicit QRImageWidget(QWidget *parent = 0);
     QImage exportImage();
 
-public slots:
+public Q_SLOTS:
     void saveImage();
     void copyImage();
 
@@ -55,7 +55,7 @@ public:
     void setModel(OptionsModel *model);
     void setInfo(const SendCoinsRecipient &info);
 
-private slots:
+private Q_SLOTS:
     void on_btnCopyURI_clicked();
     void on_btnCopyAddress_clicked();