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/recentrequeststablemodel.h

@@ -89,7 +89,7 @@ public:
     void addNewRequest(const std::string &recipient);
     void addNewRequest(RecentRequestEntry &recipient);
 
-public slots:
+public Q_SLOTS:
     void sort(int column, Qt::SortOrder order = Qt::AscendingOrder);
     void updateDisplayUnit();