Add option to exclude CCK fields from Location field in views.

we need to use early return http://pear.php.net/manual/en/standards.bestpractices.php
```php
if (empty($this->options['exclude_cck'])) {
  parent::query();
  return;
}
$genid_field = $this->query->ensure_table('location_instance', $this->relationship) . ".genid";
$this->query->add_where(0, "$genid_field NOT LIKE '%cck%' OR $genid_field IS NULL");
```