Add tomli as part of pip dependency

Summary: It's added in https://www.internalfb.com/code/fbsource/[bacc5bee6856a4cedbe38b223d761398efbbffe1]/fbcode/executorch/build/extract_sources.py?lines=20 but we need to get that dependency.

Reviewed By: tarun292

Differential Revision: D48178194

fbshipit-source-id: 4169cdf024463caada257de3726f65d9f05a7b7a