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/intro.h

@@ -43,14 +43,14 @@ public:
      */
     static QString getDefaultDataDirectory();
 
-signals:
+Q_SIGNALS:
     void requestCheck();
     void stopThread();
 
-public slots:
+public Q_SLOTS:
     void setStatus(int status, const QString &message, quint64 bytesAvailable);
 
-private slots:
+private Q_SLOTS:
     void on_dataDirectory_textChanged(const QString &arg1);
     void on_ellipsisButton_clicked();
     void on_dataDirDefault_clicked();