Add WorkspaceSettings.xcsettings for stocks app (#26993)

This file gets generated whenever you `flutter run` the stocks app and dirties the tree. Looks like we are checking those files in for the other example apps as well, e.g. https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/ios/Runner.xcworkspace/xcshareddata/WorkspaceSettings.xcsettings