Add some type hints to unicode2hex as well, it is used by
ByteArrayData.
Task-number: QTBUG-128634
Change-Id: I86b7ce8567483bf8a4d4db78c9585652526cb90d
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
(cherry picked from commit c1ffb6e7515621a953b5ebf19c384b27df785d73)