Bug 1510490 - Follow-up to fix Btup build bustage. r=bholley

Differential Revision: https://phabricator.services.mozilla.com/D13604

python/mozbuild/mozbuild/backend/cargo_build_defs.py

@@ -149,4 +149,8 @@ cargo_extra_outputs = {
     'baldrdash': [
         'bindings.rs',
     ],
+    'typenum': [
+        'op.rs',
+        'consts.rs',
+    ],
 }