[CSharpBinding] Fix Export attribute not generated for iOS project

Fixed bug 24985 - Export attribute not generated for iOS Classic
project
https://bugzilla.xamarin.com/show_bug.cgi?id=24985

In an iOS Classic project, generating code for an iOS protocol method,
e.g. AccessoryForRow in IUITableViewDelegate, would not add the
Export attribute. The correctly generated code is shown below:

[Export ("tableView:accessoryTypeForRowWithIndexPath:")]
public UITableViewCellAccessory AccessoryForRow (UITableView tableView, NSIndexPath indexPath)
{
    throw new NotImplementedException ();
}

The problem was that the code generator had been changed to generate
an Export attribute with an iOS Unified namespace (Foundation). Now
the code generator will try to determine which namespace to use by
looking at the protocol member so that both iOS Classic and iOS
Unified projects are supported.