To generate protobufs run make-protos

  $ ./make-protos
  signal:~/src/ringrtc/src/rust$ ./make-protos
     Compiling proc-macro2 v0.4.27
     Compiling unicode-xid v0.1.0
     ...
     ...
     Compiling ringrtc v0.1.0 (~/src/ringrtc/src/rust)
         Finished dev [unoptimized + debuginfo] target(s) in 22.11s
          Running `/tmp/ringrtc-protobuf/debug/protobuf-gen`
     Compiling protobufs ...
     Success: protobufs generated in src/protobuf.

The results are put in src/protobuf.  Commit the generated .rs file(s)
whenever generating new protobuf implementations.
