Cargo.toml: Add deflate and pnet to workspace

[mxinden]

#1890 reports libp2p-pnet to use an outdated dependency. More specifically salsa20 v0.6.0 instead of v0.7.0. With #1744 Dependabot should detect these outdated dependencies and propose a pull request.

libp2p-pnet (as well as libp2p-deflate) was not part of the workspace list in the root Cargo.toml. By adding libp2p-pnet (as well as libp2p-deflate) to the list Dependabot should now check libp2p-pnet as well.

//CC @elenaf9