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/trafficgraphwidget.h

@@ -27,7 +27,7 @@ public:
 protected:
     void paintEvent(QPaintEvent *);
 
-public slots:
+public Q_SLOTS:
     void updateRates();
     void setGraphRangeMins(int mins);
     void clear();