[build] Put generated.h back in snapshot dir

This is where users of the flatbuffer build rule expects snapshots are
stored. For example:

This is a followup to 194045e, didn't notice this problem until a
manual roll attempt failed CQ. Not sure why it didn't manifest when I
was testing locally, most likely due to I'm not doing a proper clean.
Did a more thorough local test this time.

rm -rf out
fx set core.x64 \
  --with //bundles:build_infra,//bundles:test_infra \
fx build

Change-Id: I972a5ba6e48f1bed883b3896ba0dc6218529bcdb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/472661
Reviewed-by: Shai Barack <shayba@google.com>
1 file changed