[BUGFIX] Cannot call static method to add flash message

Change-Id: I3fb8aac632e3e3d3e10eb3d41681ea76d3614d37
Reviewed-on: http://review.typo3.org/34316
Reviewed-by: Xavier Perseguers <xavier@typo3.org>
Tested-by: Xavier Perseguers <xavier@typo3.org>