fix use of undefined variable $summary in field formatter plugin

Intialized a $summary variable so that it doesn't throw php notice:

```Notice: Undefined variable: summary in wetkit_widgets_field_formatter_settings_summary() (line 172 of /data/www/wet/profiles/wetkit/modules/custom/wetkit_widgets/wetkit_widgets.plugins.inc).```

Offending code:
```$summary .= "Destination image style: {$value}<br />";```