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/qvaluecombobox.cpp

@@ -27,5 +27,5 @@ void QValueComboBox::setRole(int role)
 
 void QValueComboBox::handleSelectionChanged(int idx)
 {
-    emit valueChanged();
+    Q_EMIT valueChanged();
 }