Don't lint /webroot/rsrc/externals/.

Summary: If I understand correctly, the `/webroot/rsrc/externals/` directory should be treated similarly to `/externals/` in terms of linting and coding style.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9689

.arclint

@@ -1,6 +1,7 @@
 {
   "exclude": [
     "(^externals/)",
+    "(^webroot/rsrc/externals/)",
     "(/__tests__/data/)"
   ],
   "linters": {