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.cpp

@@ -139,10 +139,10 @@ void TrafficGraphWidget::updateRates()
     }
 
     float tmax = 0.0f;
-    foreach(float f, vSamplesIn) {
+    Q_FOREACH(float f, vSamplesIn) {
         if(f > tmax) tmax = f;
     }
-    foreach(float f, vSamplesOut) {
+    Q_FOREACH(float f, vSamplesOut) {
         if(f > tmax) tmax = f;
     }
     fMax = tmax;