[BUGFIX] sys_category table not listed in allowed excludefields

Enables the security flag ignoreRootLevelRestriction in order
for the sys_category table to be listed in allowed excludefields.

Resolves: #53201
Releases: 6.2, 6.1
Change-Id: If2d1ef7fb7d1af943f2ccc13f643014c20b49974
Reviewed-on: https://review.typo3.org/25268
Reviewed-by: Francois Suter
Tested-by: Francois Suter
Reviewed-by: Xavier Perseguers
Tested-by: Xavier Perseguers
Reviewed-on: https://review.typo3.org/27027