Bug 1561102 - Add license exception for the mach crate. r=chmanchester

Differential Revision: https://phabricator.services.mozilla.com/D35743

--HG--
extra : moz-landing-system : lando

python/mozbuild/mozbuild/vendor_rust.py

@@ -185,6 +185,7 @@ Please commit or stash these changes before vendoring, or re-run with `--ignore-
             'arrayref',
             'cloudabi',
             'Inflector',
+            'mach',
         ],
         'BSD-3-Clause': [
         ]