[proposal] enable libssh on graphics/ffmpeg

Hello,

It is not currently possible to display videos through SSH using ffmpeg
based-software, mpv for example. The attached patch enables libssh
within ffmpeg. With this, I can for example browse a remote server using
sftp URL in Thunar and select mpv to display the video.

I briefly tested it using ffmpeg versions from 7.8 and -snapshot ports
tree. The attached patch is for ffmpeg 8 on the -snapshot branch. I can
also provide the patch for ffmpeg 6 on the -7.8 branch if needed.

Hope it helps,
Joel C.