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/walletmodel.h

@@ -227,7 +227,7 @@ private:
     void unsubscribeFromCoreSignals();
     void checkBalanceChanged();
 
-signals:
+Q_SIGNALS:
     // Signal that balance in wallet changed
     void balanceChanged(const CAmount& balance, const CAmount& unconfirmedBalance, const CAmount& immatureBalance,
                         const CAmount& watchOnlyBalance, const CAmount& watchUnconfBalance, const CAmount& watchImmatureBalance);
@@ -252,7 +252,7 @@ signals:
     // Watch-only address added
     void notifyWatchonlyChanged(bool fHaveWatchonly);
 
-public slots:
+public Q_SLOTS:
     /* Wallet status might have changed */
     void updateStatus();
     /* New transaction, or transaction changed status */