[xnnpack][lite-int][1/n] flatbuffer buck rules (#87826)
Writing a placeholder schema.fbs file for now to setup the buck gen rules. The generated schema file will be used in the xnnpack name space and be reserved for serialization/deserialization of our xnnpack lowered graph
Steps Accomplished
- Buck rules to compile flatbuffer schema
- added header file to preprocess
- everything compiles correctly
Differential Revision: [D38999169](https://our.internmc.facebook.com/intern/diff/D38999169/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D38999169/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87826
Approved by: https://github.com/digantdesai
