vtkFFMPEGWriter: Fix build with libavcodec55

Version 55 of the libavcodec API adds an AV_ prefix to the CODEC_ID_
constants.  Select a constant name based on the API version in use.

Change-Id: I6497745dd8db9939b4f9c929f587beb5a128167a
Merge topic 'vtkFFMPEGWriter-port-libavcodec55' into master

63b5bc77 vtkFFMPEGWriter: Fix build with libavcodec55