multimedia/tvheadend: unbreak with ffmpeg 3.x

src/plumbing/transcoding.c:27:10: fatal error: 'libavutil/audioconvert.h' file not found
#include <libavutil/audioconvert.h>

^

src/plumbing/transcoding.c:1064:30: error: use of undeclared identifier 'PIX_FMT_YUV420P'; did you

mean 'AV_PIX_FMT_YUV420P'?
octx->pix_fmt        = PIX_FMT_YUV420P;
                       ^~~~~~~~~~~~~~~
                       AV_PIX_FMT_YUV420P

/usr/local/include/libavutil/pixfmt.h:62:5: note: 'AV_PIX_FMT_YUV420P' declared here

AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
^

src/plumbing/transcoding.c:1092:30: error: use of undeclared identifier 'PIX_FMT_YUV420P'; did you

mean 'AV_PIX_FMT_YUV420P'?
octx->pix_fmt        = PIX_FMT_YUV420P;
                       ^~~~~~~~~~~~~~~
                       AV_PIX_FMT_YUV420P

/usr/local/include/libavutil/pixfmt.h:62:5: note: 'AV_PIX_FMT_YUV420P' declared here

AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
^

src/plumbing/transcoding.c:1123:30: error: use of undeclared identifier 'PIX_FMT_YUV420P'; did you

mean 'AV_PIX_FMT_YUV420P'?
octx->pix_fmt        = PIX_FMT_YUV420P;
                       ^~~~~~~~~~~~~~~
                       AV_PIX_FMT_YUV420P

/usr/local/include/libavutil/pixfmt.h:62:5: note: 'AV_PIX_FMT_YUV420P' declared here

AV_PIX_FMT_YUV420P,   ///< planar YUV 4:2:0, 12bpp, (1 Cr & Cb sample per 2x2 Y samples)
^

build.freebsd/src/plumbing/transcoding.o: In function `transcoder_input':
src/plumbing/transcoding.c:(.text+0x559): undefined reference to `avcodec_alloc_frame'
src/plumbing/transcoding.c:(.text+0x56a): undefined reference to `avcodec_alloc_frame'
src/plumbing/transcoding.c:(.text+0x57f): undefined reference to `avcodec_get_frame_defaults'
src/plumbing/transcoding.c:(.text+0x58a): undefined reference to `avcodec_get_frame_defaults'
build.freebsd/src/plumbing/transcoding.o: In function `transcoder_stream_video':
src/plumbing/transcoding.c:(.text+0x1904): undefined reference to `avpicture_deinterlace'
cc: error: linker command failed with exit code 1 (use -v to see invocation)

PR: 214815
Reported by: antoine (via exp-run)
Obtained from: upstream
Approved by: decke (maintainer)