QFileSelector: correct the macros for Apple platforms.

This fixes a serious regression from
7d72516b52b20b0782d972224a55a43e74b8ae5a.

[ChangeLog] QFileSelector: the identifier for OS X has been changed back
to 'osx' from 'mac', and 'mac' and 'darwin' have now been added as
selectors for Darwin OS (which is the base of both OS X and iOS).

Task-number: QTBUG-35073
Change-Id: I83183e34c5a697338cc1ddcac33a41bd379ded12
Reviewed-by: Alan Alpert <aalpert@blackberry.com>