Add license header

Author: sami-daniel

src/side_diff.rs

@@ -1,3 +1,8 @@
+// This file is part of the uutils diffutils package.
+//
+// For the full copyright and license information, please view the LICENSE-*
+// files that was distributed with this source code.
+
 use crate::utils::limited_string;
 use diff::Result;
 use std::{