multimedia/libvpx: update 1.6.1

Changes: https://chromium.googlesource.com/webm/libvpx/+log/v1.6.0..v1.6.1
ACC report: https://people.freebsd.org/~jbeich/compat_reports/libvpx/1.6.0_to_1.6.1/compat_report.html
MFH: 2017Q1 (maybe security from fuzz fixes)