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/walletmodeltransaction.cpp

@@ -81,7 +81,7 @@ void WalletModelTransaction::reassignAmounts(int nChangePosRet)
 CAmount WalletModelTransaction::getTotalTransactionAmount()
 {
     CAmount totalTransactionAmount = 0;
-    foreach(const SendCoinsRecipient &rcp, recipients)
+    Q_FOREACH(const SendCoinsRecipient &rcp, recipients)
     {
         totalTransactionAmount += rcp.amount;
     }