add prerequisites

[yeoleobun]

since opus-rs have requirements from audiopus_sys, it need to know the locations of preinstalled opus headers/libraries。

By default, audiopus_sys will use pkg-config on Unix or GNU. Setting the environment variable LIBOPUS_NO_PKG or OPUS_NO_PKG will bypass probing for Opus via pkg-config.

or you need to set environment variable LIBOPUS_LIB_DIR or OPUS_LIB_DIR.