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/macdockiconhandler.h

@@ -30,7 +30,7 @@ public:
     static void cleanup();
     void handleDockIconClickEvent();
 
-signals:
+Q_SIGNALS:
     void dockIconClicked();
 
 private: