Enable linting in arc.

Summary: Just change some config.

Test Plan: arc lint

Reviewers: chip

CC: leveldb

Differential Revision: https://reviews.facebook.net/D8355

.arcconfig

 {
   "project_id" : "leveldb",
   "conduit_uri" : "https://reviews.facebook.net/",
-  "copyright_holder" : ""
+  "copyright_holder" : "",
+  "load" : [
+    "linters/src/"
+  ],
+  "lint.engine" : "FacebookFbcodeLintEngine",
+  "lint.engine.single.linter" : "FbcodeCppLinter"
 }