Make .proto files generated by "piqi to-proto" as "proto2"

"piqi to-proto" is now generating 'syntax = "proto2"'; at the top of
converted .proto files. This prevents protoc from printing warnings like
this:

    No syntax specified for the proto file: ... Please use 'syntax =
    "proto2";' or 'syntax = "proto3";' to specify a syntax version.
    (Defaulted to proto2 syntax.)

src/to_proto.ml

@@ -464,6 +464,7 @@ let gen_piqi (piqi:T.piqi) =
     List.map (fun x -> iol [ios x; eol; eol]) piqi.P.protobuf_custom
   in
   iol [
+    ios "syntax = \"proto2\";\n\n";
     package;
     iol proto_custom;
     piqi_import;