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/qvalidatedlineedit.h

@@ -27,11 +27,11 @@ private:
     bool valid;
     const QValidator *checkValidator;
 
-public slots:
+public Q_SLOTS:
     void setValid(bool valid);
     void setEnabled(bool enabled);
 
-private slots:
+private Q_SLOTS:
     void markValid();
     void checkValidity();
 };