MFH: r411161

multimedia/libav: update to 11.6

As upstream ignores many FFmpeg CVE fixes I'm not adjusting VuXML to
err on the side of security paranoia.

Changes: https://git.libav.org/?p=libav.git;a=blob;f=Changelog;hb=refs/tags/v11.6
Approved by: ports-secteam (feld)