Add Qt_DefaultCompiledVersion to the QDataStream::Version enum.

This allows code to, amongst other things, use the latest compiled
QDataStream version in static const variables, where previously the
only alternative was to hard-code the version (and update it with every
Qt version bump) or use QDataStream().version(), which is not allowed
because it increases Qt's startup time.

Change-Id: If1f29fc9cd83c9d7fe619a4ca197152aa9c77a04
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>