Fix llama2 buck targets (#1941)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1941

Fix 2 targets in fbcode CI jobs (didn't repro in dev environment):

* fbcode//executorch/examples/models/llama2:export_llama
* fbcode//executorch/examples/models/llama2/fb:stories110M_test

See example failure:

https://www.internalfb.com/sandcastle/workflow/1490691476661671339/artifact/actionlog.1490691476702112874.stdout.1?selectedLines=1060-1060-48-102

Reviewed By: lucylq

Differential Revision: D53650711

fbshipit-source-id: 95a469e8942eb629d53a7c358b0a506322e9982b