graphics/opencv: unbreak with ffmpeg 4.0

In file included from modules/highgui/src/cap_ffmpeg.cpp:45:
modules/highgui/src/cap_ffmpeg_impl.hpp:1481:21: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

c->flags |= CODEC_FLAG_GLOBAL_HEADER;
            ^

modules/highgui/src/cap_ffmpeg_impl.hpp:1505:30: error: use of undeclared identifier 'AVFMT_RAWPICTURE'

if (oc->oformat->flags & AVFMT_RAWPICTURE) {
                         ^

modules/highgui/src/cap_ffmpeg_impl.hpp:1679:35: error: use of undeclared identifier 'AVFMT_RAWPICTURE'

if( (oc->oformat->flags & AVFMT_RAWPICTURE) == 0 )
                          ^

modules/highgui/src/cap_ffmpeg_impl.hpp:1913:32: error: use of undeclared identifier 'AVFMT_RAWPICTURE'

if (!(oc->oformat->flags & AVFMT_RAWPICTURE)) {
                           ^

modules/highgui/src/cap_ffmpeg_impl.hpp:2207:25: error: use of undeclared identifier 'CODEC_FLAG_GLOBAL_HEADER'

c->flags |= CODEC_FLAG_GLOBAL_HEADER;
            ^

PR: 227726
Obtained from: upstream (via Homebrew)