vcpu: Fix import for later protobuf releases.
vcpu: Fix import for later protobuf releases.

Unblock the rust toolchain upgrade by unblocking the protobuf upgrade.

Later versions of dev-rust/protobuf do not export CodedOutputStream from
protobuf::stream anymore.

BUG=b:191365098
TEST=cargo build

Change-Id: I21c76605eb16a3b456f3608afbb0a7c507306d78
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2978853
Auto-Submit: Allen Webb <880184b418addffb87cd9c8e8e882ca6764ac96b@google.com>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

google/crosvm,google/crosvm,google/crosvm,google/crosvm