contrib: bump opus to 1.2

Opus 1.2 brings many improvements, including
speech quality improvements, performance improvements,
and support for Ambisonics mapping, that could be used
for VR.

See https://people.xiph.org/~jm/opus/opus-1.2/

Using latest Opus has no major implication but
brings "free" quality and performance improvements and
is good for marketing purposes.

Change-Id: Ib7fa3ed17e423fffa8ac79f8fe8e3af46057991c