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/splashscreen.h

@@ -27,7 +27,7 @@ protected:
     void paintEvent(QPaintEvent *event);
     void closeEvent(QCloseEvent *event);
 
-public slots:
+public Q_SLOTS:
     /** Slot to call finish() method as it's not defined as slot */
     void slotFinish(QWidget *mainWin);