add getpgid in changelog

Author: kiddkai

CHANGELOG.md

@@ -6,6 +6,8 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 ### Changed
+- Added `getpgid` in `::nix::unistd`
+  ([#433](https://github.com/nix-rust/nix/pull/433))
 - `pipe2` now calls `libc::pipe2` where available. Previously it was emulated
   using `pipe`, which meant that setting `O_CLOEXEC` was not atomic.
   ([#427](https://github.com/nix-rust/nix/pull/427))