Cargo.toml: Specify protobuf = 2.0

This prevents cargo from selecting a newer major release which
is incompatible. Specifying the version would unblock adding
protobuf 3.2 to ChromeOS' crate registry.

Bug: 262404106
Test: manual - emerge-amd64-generic third-party-crates-src floss with changes applied in ChromeOS' tree
Change-Id: I170de01cf1e7c10683cc0d0fe673dc90092f35f1