[dune] ignore all dirs except src/

Summary:
This avoids dune scanning 2000+ directories (according to its logs),
mostly due to scanning infer/tests/ I think.

Reviewed By: artempyanykh

Differential Revision: D20798915

fbshipit-source-id: 3764cd3fb

infer/dune

@@ -0,0 +1,5 @@
+; Copyright (c) Facebook, Inc. and its affiliates.
+;
+; This source code is licensed under the MIT license found in the
+; LICENSE file in the root directory of this source tree.
+(dirs src bin)