[BUGFIX] Missing static keyword for addFieldsToUserSettings

All t3lib_extMgm methods are meant to be called statically.
The function addFieldsToUserSettings is missing the static keyword.

Change-Id: I4dbc1f64bc432695fac8a295261e53560bcb03d7
Fixes: #36399
Releases: 4.5,4.6,4.7,6.0
Reviewed-on: http://review.typo3.org/10669
Reviewed-by: Tolleiv Nietsch
Tested-by: Tolleiv Nietsch
Reviewed-by: Oliver Klee
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn