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/coincontroldialog.cpp

@@ -461,7 +461,7 @@ void CoinControlDialog::updateLabels(WalletModel *model, QDialog* dialog)
     CAmount nPayAmount = 0;
     bool fDust = false;
     CMutableTransaction txDummy;
-    foreach(const CAmount &amount, CoinControlDialog::payAmounts)
+    Q_FOREACH(const CAmount &amount, CoinControlDialog::payAmounts)
     {
         nPayAmount += amount;