Update for new signature of get_root_internal().
Update for new signature of get_root_internal().

tempbottle/capnproto-rust,dwrensha/capnproto-rust,placrosse/capnp-rpc-rust,placrosse/capnp-rpc-rust,placrosse/capnproto-rust,dwrensha/capnp-rpc-rust,tempbottle/capnp-rpc-rust,tempbottle/capnp-rpc-rust,dwrensha/capnp-rpc-rust