[BUGFIX] Add SoftIndex parser typolink to link in sys_file_reference

The field "link" of the table "sys_file_reference" handles links. To
make the SoftReferenceIndex, which is used by impexp and
linkvalidator, work right, the “softref” key "typolink" should be
registered.

Resolves: #57010
Releases: 6.2, 6.1, 6.0
Change-Id: I95cb8dbfa0c6d84926cd57d1fe4e4dad93e9c21a
Reviewed-on: https://review.typo3.org/28807
Reviewed-by: Marc Bastian Heinrichs
Tested-by: Marc Bastian Heinrichs