libopus: 1.2 -> 1.2.1

> http://opus-codec.org/release/stable/2017/06/26/libopus-1_2_1.html

This Opus 1.2.1 minor release fixes a relatively rare issue where the 1.2 encoder would wrongly assume a signal to be bandlimited to 12 kHz and not encode frequencies between 12 and 20 kHz.
This only happens on a few clips, but it is good to update to avoid a potential loss of quality.

There are no other changes compared to 1.2.