[BUGFIX] Visibility of method tx_sysaction_task::fixUsername()

The method fixUsername() is defined as private, but should be
protected instead.

Change-Id: I0513416a0d1cf07793708e86e7f1c7fde619f4d5
Resolves: #25219
Releases: 4.6, 4.5, 4.4
Reviewed-on: http://review.typo3.org/1104
Reviewed-by: Jacob Rasmussen
Tested-by: Jacob Rasmussen
Reviewed-by: Oliver Klee
Reviewed-by: Andy Grunwald
Tested-by: Andy Grunwald
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn