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

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

Change-Id: Ic5a8b54db751661a421385bcbccc264e01e958d3
Resolves: #38601
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/16753
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer