[BUGFIX] Fix Non-static method tslib_cObj::enableFields()

Non-static method tslib_cObj::enableFields() should
not be called statically

Change-Id: I93497b24618d29534331ac42d87ec2baec91cad6
Resolves: #38602
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/16751
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer