[BUGFIX] t3lib_div should contain only static methods

A few methods don't have the static keyword. This leads to PHP notices.

Change-Id: I90a99601a39cd26abed665b42a8b054385f561ff
Fixes: #30050
Releases: 4.6
Reviewed-on: http://review.typo3.org/5075
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers