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/receivecoinsdialog.h

@@ -44,7 +44,7 @@ public:
 
     void setModel(WalletModel *model);
 
-public slots:
+public Q_SLOTS:
     void clear();
     void reject();
     void accept();
@@ -60,7 +60,7 @@ private:
     void copyColumnToClipboard(int column);
     virtual void resizeEvent(QResizeEvent *event);
 
-private slots:
+private Q_SLOTS:
     void on_receiveButton_clicked();
     void on_showRequestButton_clicked();
     void on_removeRequestButton_clicked();