accounts/usbwallet/trezor: expose protobuf package (#17980)

When some of the same messages are redefined anywhere in a Go project,
the protobuf package panics (see
https://github.com/golang/protobuf/issues/178).

Since this package is internal, there is no way to work around it, as
one cannot use it directly, but also cannot define the same messages.

There is no downside in making the package accessible.