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/receivecoinsdialog.cpp

@@ -185,7 +185,7 @@ void ReceiveCoinsDialog::on_showRequestButton_clicked()
         return;
     QModelIndexList selection = ui->recentRequestsView->selectionModel()->selectedRows();
 
-    foreach (const QModelIndex& index, selection) {
+    Q_FOREACH (const QModelIndex& index, selection) {
         on_recentRequestsView_doubleClicked(index);
     }
 }