QtHelp: Don't truncate the size of registered documentation files

The size of a registered .qch file is stored in the collection file
to check whether the index data is up to date. It was read back into
an int, so for files larger than 2 GiB it never matched the actual
file size, and the documentation was indexed again every time the
collection file was opened.

Use qint64 for the size.

Task-number: QTBUG-150686
Pick-to: 6.12 6.8
Change-Id: Ida0faf550ffec47abaedb6ac1468393ce148033f
Reviewed-by: Kai Köhne <kai.koehne@qt.io>