Use v2 cargo resolver

The 2021 usually implies resolver v2, and warns otherwise.

Author: nyurik

Cargo.toml

@@ -1,4 +1,5 @@
 [workspace]
+resolver = "2"
 members = [
     "bindgen",
     "bindgen-cli",
@@ -7,7 +8,6 @@ members = [
     "bindgen-tests/tests/quickchecking",
     "bindgen-tests/tests/expectations",
 ]
-
 default-members = [
     "bindgen",
     "bindgen-cli",
@@ -54,4 +54,3 @@ release = false
 [profile.dist]
 inherits = "release"
 lto = "thin"
-

bindgen/lib.rs

@@ -74,6 +74,8 @@ use std::path::{Path, PathBuf};
 use std::process::{Command, Stdio};
 use std::rc::Rc;
 use std::str::FromStr;
+
+#[cfg(feature = "runtime")]
 use std::sync::{Arc, OnceLock};
 
 // Some convenient typedefs for a fast hash map and hash set.