databases/grass: unbreak build with ffmpeg 3.x

lib/ogsf/gsd_img_mpeg.c:41:24: error: 'PIX_FMT_YUV420P' undeclared (first use in this function)
#define STREAM_PIX_FMT PIX_FMT_YUV420P /* default pix_fmt */

^

lib/ogsf/gsd_img_mpeg.c:97:18: note: in expansion of macro 'STREAM_PIX_FMT'

c->pix_fmt = STREAM_PIX_FMT;
             ^

lib/ogsf/gsd_img_mpeg.c:41:24: note: each undeclared identifier is reported only once for each function it appears in
#define STREAM_PIX_FMT PIX_FMT_YUV420P /* default pix_fmt */

^

lib/ogsf/gsd_img_mpeg.c:97:18: note: in expansion of macro 'STREAM_PIX_FMT'

c->pix_fmt = STREAM_PIX_FMT;
             ^

lib/ogsf/gsd_img_mpeg.c:98:24: error: 'CODEC_ID_MPEG2VIDEO' undeclared (first use in this function)

if (c->codec_id == CODEC_ID_MPEG2VIDEO) {
                   ^

PR: 207547