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.h

@@ -24,13 +24,13 @@ public:
     /** Specify model role to use as ordinal value (defaults to Qt::UserRole) */
     void setRole(int role);
 
-signals:
+Q_SIGNALS:
     void valueChanged();
 
 private:
     int role;
 
-private slots:
+private Q_SLOTS:
     void handleSelectionChanged(int idx);
 };