Add missing std feature to allow to be used as dependency of proc-macro

[GuillaumeGomez]

This allows to fix this error when building proc-macro:

error[E0463]: can't find crate for `std`
  |
  = note: the `x86_64-unknown-linux-gnu` target may not be installed
  = help: consider downloading the target with `rustup target add x86_64-unknown-linux-gnu`
  = help: consider building the standard library from source with `cargo build -Zbuild-std`

r? @Urgau