webrtc-rs: update version to v0.8.0

[dasxran]

Current version is webrtc = "=0.2.1" and interceptor = "=0.4.0"

error[E0599]: no function or associated item named `unmarshal` found for struct `rtp::header::Header` in the current scope
   --> .cargo\registry\src\index.crates.io-6f17d22bba15001f\webrtc-srtp-0.7.1\src\session\mod.rs:150:39
    |
150 |             vec![rtp::header::Header::unmarshal(&mut buf)?.ssrc]
    |                                       ^^^^^^^^^ function or associated item not found in `Header`
    |
    = help: items from traits can only be used if the trait is in scope
help: the following trait is implemented but not in scope; perhaps add a `use` for it:
    |
1   + use webrtc_util::marshal::Unmarshal;

[sipsorcery]

PR merged.