net/freerdp: Fix build with ffmpeg 3.0.x

Add patch-ffmpeg29, obtained from Debian [1], 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.

[1] http://sources.debian.net/patches/freerdp/1.1.0%7Egit20140921.1.440916e%2Bdfsg1-10/1011_ffmpeg-2.9.patch/

Approved by: adamw (mentor)
Differential Revision: https://reviews.freebsd.org/D6813