fbcode_builder: add a license header to FBBuildOptions.cmake
fbcode_builder: add a license header to FBBuildOptions.cmake

Summary:
Add a license header to satisfy the open source linter. Use the same
header the other .cmake files have.

Reviewed By: mhlakhani

Differential Revision: D17404782

fbshipit-source-id: 66679d72c9e680f8bb8b27869e981a046b3520cf

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst