MCPClient control: specify libavcodec-extra-56 dependency
MCPClient control: specify libavcodec-extra-56 dependency

libvo_aacenc requires ffmpeg to be built as GPL3, and won't be
available in the default libavcodec build. Since faac and libfdk_aac
are both nonfree, this means that ffmpeg isn't installed with any AAC
encoder other than the experimental internal one by default.

artefactual/archivematica,artefactual/archivematica,michal-ruzicka/archivematica,artefactual/archivematica,sevein/archivematica,eckardm/archivematica,eckardm/archivematica,eckardm/archivematica,michal-ruzicka/archivematica,sevein/archivematica,michal-ruzicka/archivematica,artefactual/archivematica,michal-ruzicka/archivematica,sevein/archivematica,eckardm/archivematica,sevein/archivematica