Migrate libc-test to 2018 edition

edition: cannot update to 2021 edition because of libc msrv.
cargo of rust 1.19 cannot understand edition key in cargo.toml.

Author: tesuji

libc-test/Cargo.toml

@@ -1,6 +1,7 @@
 [package]
 name = "libc-test"
 version = "0.2.155"
+edition = "2018"
 authors = ["The Rust Project Developers"]
 license = "MIT OR Apache-2.0"
 build = "build.rs"