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/peertablemodel.h

@@ -68,7 +68,7 @@ public:
     void sort(int column, Qt::SortOrder order);
     /*@}*/
 
-public slots:
+public Q_SLOTS:
     void refresh();
 
 private: