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/addressbookpage.cpp

@@ -254,7 +254,7 @@ void AddressBookPage::done(int retval)
     // Figure out which address was selected, and return it
     QModelIndexList indexes = table->selectionModel()->selectedRows(AddressTableModel::Address);
 
-    foreach (const QModelIndex& index, indexes) {
+    Q_FOREACH (const QModelIndex& index, indexes) {
         QVariant address = table->model()->data(index);
         returnValue = address.toString();
     }