[BUGFIX] Fatal error in indexed_search

FileContentParser misses one backslash for RuntimeException
to be in global namespace.
Only master is affected, since backports got corrected already.

Releases: 6.2
Resolves: #55128
Change-Id: I015c1236847c4963dd28f400d14a5fe97b7868e6
Reviewed-on: https://review.typo3.org/26922
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Markus Klein
Tested-by: Markus Klein