Make utils::execute cross-platform

Make utils::execute cross-platform so that the project can hopefully be
built on windows as well as unix based platforms in the future.

This relates to issue #218

[changelog]
changed: utils::execute to build on both unix & windows platforms

<!-- ps-id: 6dec3d61-898a-40d5-b0aa-b537d97c3a30 -->

src/ps/private/utils/execute.rs

@@ -1,5 +1,8 @@
 use std::io;
-use std::os::unix::prelude::ExitStatusExt;
+#[cfg(target_family = "unix")]
+use std::os::unix::process::ExitStatusExt;
+#[cfg(target_family = "windows")]
+use std::os::windows::process::ExitStatusExt;
 use std::process::{Command, Output};
 use std::result::Result;