[build] Fix hermeticity of flatbuffer

Change-Id: Ib0bde92dcf9397d80c2ae064ed3cc0cd63efc04e
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/472637
Reviewed-by: Shai Barack <shayba@google.com>
1 file changed