net/freerdp: Fix build with ffmpeg 3.0.x
Add patch-ffmpeg29, obtained from Debian , to allow freerdp 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.
Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D6813