Ignore licenselint for llama2 example in Executorch

Summary:
llama2 as its own specific license. Will ignore the fbcode-wide licenselint for that file.

https://www.internalfb.com/code/fbsource/[def2f612b7200f78b46041779e8ee7f13b76e247]/fbcode/executorch/examples/models/llama2/model.py

Reviewed By: zertosh

Differential Revision: D51164249

fbshipit-source-id: 54f4c3c6d2d4d5325509eff122634f65560d2957