libav contrib bump

Bumps up libav to match the FFmpeg bump. Includes many bug fixes. Full
list of changes here:
https://git.libav.org/?p=libav.git;a=blob;f=Changelog;hb=refs/tags/v12.1



Change-Id: Ieca119af8d56eed45184378080c710285fb60b1c
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>

contrib/src/libav/rules.mak

-#Libav 11-1 (git version packaged for ubuntu  14.10)
-LIBAV_HASH := f851477889ae48e2f17073cf7486e1d5561b7ae4
+LIBAV_HASH := v12.1
 LIBAV_URL := https://git.libav.org/?p=libav.git;a=snapshot;h=$(LIBAV_HASH);sf=tgz
 
 ifndef HAVE_LINUX