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

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

Change-Id: I9d6b5ac1c4e97164af8a3ce0eaaa0fe95c4a6860
Resolves: #38600
Releases: 6.1, 6.0
Reviewed-on: https://review.typo3.org/16754
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Stefan Neufeind
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer