[BUGFIX] Modification date comparison fixed in FAL indexer

modification_date is a string from the database and SplFileInfo
mtime is returned as integer. Cast modification_date to
an integer too.

Resolves: #75421
Releases: master,7.6
Change-Id: I7523f9f722af8cc5f4e137e32bf07e2c3105214d
Reviewed-on: https://review.typo3.org/47519


Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Tested-by: Andreas Fernandez <typo3@scripting-base.de>