[BUGFIX] Fix wrong class name for "ThumbnailProcessor"

Change-Id: Id33c9f10a762806d9ba3dd20515877f13387c6b1
Releases: 0.5
Resolves: #61606
Reviewed-on: http://review.typo3.org/32779
Reviewed-by: Fabien Udriot <fabien.udriot@ecodev.ch>
Tested-by: Fabien Udriot <fabien.udriot@ecodev.ch>