[BUGFIX] Stylesheet for confirmation view missing in form sysext

The confirmation message does not have any CSS at the moment.

Change-Id: Ic2e182cda9f5333e0c6d7aad4d4bb9f929ca0568
Fixes: #13214
Reviewed-on: http://review.typo3.org/6071
Reviewed-by: Xavier Perseguers
Reviewed-by: Oliver Hader
Tested-by: Oliver Hader