[BUGFIX] Respect gridelements in the Content Object Loader

Add the gridelements fields only if there is a gridelements extension
is loaded.

Change-Id: I94cfb4191e88748bf1c41ee5ef462f3819ac9ab3
Reviewed-on: https://review.typo3.org/31531
Reviewed-by: Tim Spiekerkötter
Tested-by: Tim Spiekerkötter