QFutureCallOutEvent is used externally, so it needs to be exported.
This reverts commit 3141a13b2a.
Fixes: QTBUG-104732
Pick-to: 6.4
Change-Id: I82c9e7414192ee948f78259bd74a404691a7805a
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Marc Mutz <marc.mutz@qt.io>