[BUGFIX] PHP Runtime Notice in saltedpasswords

Declaration of method compareUident() should be compatible with its
parent class.

Change-Id: I66979bd76b204bbb762c41037cdaae5f006debb5
Fixes: #56769
Releases: 6.2
Reviewed-on: https://review.typo3.org/28261
Reviewed-by: Andreas Fernandez
Tested-by: Andreas Fernandez
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters