lavc: add a dummy field to AVStream to preserve ABI compatibility for avconv

avconv abuses the API by accessing AVStream.parser (which is private).
Removing AVStream.reference_dts in
2ba68dd044ca8fc591139c05563840f546a9c0c0 breaks ABI compatibility for an
old avconv using a newer lavf. Fix this by adding a dummy field until
the next bump.
avconv: stop accessing AVStream.parser

It is private and must not be touched from outside of lavf.