contrib: fix building libvpx on mingw


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6166 b64f7644-9d1e-0410-96f1-a4d463321fa5
contrib: fix ffmpeg dependencies to include libvpx


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6167 b64f7644-9d1e-0410-96f1-a4d463321fa5