net/opal: Fix build with ffmpeg 3.0.x

• Add patch to allow opal to build successfully once multimedia/ffmpeg is updated to 3.0.x. Note that multimedia/ffmpeg is currently still on the 2.8.x branch, however this patch is backwards compatible with ffmpeg 2.8.x.

PR: 209346
Reported by: antoine (PR207547)
Reviewed by: kwm (with gnome hat)
Approved by: adamw (mentor)
Obtained from: ArchLinux [1]
Differential Revision: https://reviews.freebsd.org/D6308

[1] https://git.archlinux.org/svntogit/packages.git/tree/trunk/opal-ffmpeg3.patch?h=packages/opal