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.
Merge commit '8b64c2ba0382892cad9e1a5ba601696d4cbb4d04'

* commit '8b64c2ba0382892cad9e1a5ba601696d4cbb4d04':
  lavc: add a dummy field to AVStream to preserve ABI compatibility for avconv

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avutil/opt: fix flags check on non x86

This should fix several fate failures

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>