[Qt] constify foreach uses where possible

- this doesn't replace BOOST_FOREACH, it just makes used arguments const
  where possible

src/qt/addressbookpage.cpp

@@ -254,8 +254,7 @@ void AddressBookPage::done(int retval)
     // Figure out which address was selected, and return it
     QModelIndexList indexes = table->selectionModel()->selectedRows(AddressTableModel::Address);
 
-    foreach (QModelIndex index, indexes)
-    {
+    foreach (const QModelIndex& index, indexes) {
         QVariant address = table->model()->data(index);
         returnValue = address.toString();
     }