[BUGFIX] AbstractBackendViewHelper uses namespaces

Namespaces are not supported in PHP 5.2.x, hence one must not
prefix a class name with backslash.

Regression fix to #54748.

Resolves: #59825
Releases: 4.5
Change-Id: Ideb2cef1c5e2ec0d2ac3328ebd4f318a161d368a
Reviewed-on: https://review.typo3.org/31084
Tested-by: Sebastian Sommer
Tested-by: Steffen Mächtel
Reviewed-by: Markus Klein
Tested-by: Markus Klein