Fix form-control-feedback position when label has sr-only class

Fixes feedback positioning in the case where the label contained within a form group has the `sr-only` class. Example HTML:
```html
<div class="form-group has-feedback">
	<label class="control-label sr-only">Label</label>
	<input type="text" class="form-control">
	<span class="glyphicon glyphicon-ok form-control-feedback"></span>
</div>
```
[This jsbin](http://jsbin.com/nadaveju/1/) has a few form-group variations, the first of which illustrates the bug. [This jsbin](http://jsbin.com/nadaveju/2/) uses the same HTML but adds my proposed CSS fix.

This case was mentioned in issue #12873.