Add entry to CHANGELOG about OpenBSD fixes

Author: wezm

CHANGELOG.md

@@ -20,6 +20,10 @@ This project adheres to [Semantic Versioning](http://semver.org/).
   It accepts a `None` value for default protocol that was specified with zero using `c_int`.
   ([#647](https://github.com/nix-rust/nix/pull/647))
 
+# Fixed
+- Fix compilation and tests for OpenBSD targets
+  ([#688](https://github.com/nix-rust/nix/pull/688))
+
 ## [0.9.0] 2017-07-23
 
 ### Added