x509-cert: std feature implies const-oid/std

[woodruffw]

Just a small thing, noticed while trying to use the Error trait through a re-export here.

[woodruffw]

Without this, x509-cert/std doesn't re-export const-oid/std's gated Error implementation:

   Compiling ctltool v0.1.0 (/Users/william/devel/windows-ctl/ctltool)
error[E0277]: the trait bound `x509_cert::der::const_oid::Error: std::error::Error` is not satisfied
   --> ctltool/src/main.rs:107:44
    |
107 |         .collect::<Result<HashSet<_>, _>>()?;
    |                                            ^ the trait `std::error::Error` is not implemented for `x509_cert::der::const_oid::Error`
    |
    = help: the following other types implement trait `FromResidual<R>`:
              <Result<T, F> as FromResidual<Result<Infallible, E>>>
              <Result<T, F> as FromResidual<Yeet<E>>>
    = note: required for `anyhow::Error` to implement `From<x509_cert::der::const_oid::Error>`
    = note: required for `Result<(), anyhow::Error>` to implement `FromResidual<Result<Infallible, x509_cert::der::const_oid::Error>>`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `ctltool` due to previous error

[tarcieri]

Thanks!