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/transactiontablemodel.h

@@ -98,7 +98,7 @@ private:
     QVariant txWatchonlyDecoration(const TransactionRecord *wtx) const;
     QVariant txAddressDecoration(const TransactionRecord *wtx) const;
 
-public slots:
+public Q_SLOTS:
     /* New transaction, or transaction changed status */
     void updateTransaction(const QString &hash, int status, bool showTransaction);
     void updateConfirmations();