latest nightly error: `extern crate` is not idiomatic in the new edition

[omac777]

When using the latest rust nightly:

cargo --version --verbose
cargo 1.42.0-nightly (86134e766 2019-12-23)
release: 1.42.0
commit-hash: 86134e7666a088682f20b76278c3ee096a315218
commit-date: 2019-12-23

and building vgtk:

cargo build --release

produces the following error:

error: `extern crate` is not idiomatic in the new edition
 --> macros/src/lib.rs:4:1
  |
4 | extern crate proc_macro;
  | ^^^^^^^^^^^^^^^^^^^^^^^^ help: convert it to a `use`

The following fixes it:

use proc_macro;