Explicitly set the edition to 2015

This just suppresses a warning. Ideally this should go to 2021 but that
requires quite a bit of refactoring, so hold off until the rest of the
0.2 cleanup is complete.

(apply <rust-lang#4058> to `main`)
(cherry picked from commit 1e88f41)

Author: tgross35

Cargo.toml

@@ -4,6 +4,7 @@ version = "0.2.151"
 authors = ["The Rust Project Developers"]
 license = "MIT OR Apache-2.0"
 readme = "README.md"
+edition = "2015"
 repository = "https://github.com/rust-lang/libc"
 homepage = "https://github.com/rust-lang/libc"
 documentation = "https://docs.rs/libc/"