[build] Put generated.h back in snapshot dir

This is where users of the flatbuffer build rule expects snapshots are
stored. For example:
https://cs.opensource.google/fuchsia/fuchsia/+/master:src/ui/scenic/lib/gfx/snapshot/serializer.h;l=9;drc=ed53a56dfcb5e30e00894691ae2748636cd5685e

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.

Test:
rm -rf out
fx set core.x64 \
  --with //bundles:build_infra,//bundles:test_infra \
  --args=build_should_trace_actions=true
fx build

Change-Id: I972a5ba6e48f1bed883b3896ba0dc6218529bcdb
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/472661
Reviewed-by: Shai Barack <shayba@google.com>
diff --git a/flatbuffer.gni b/flatbuffer.gni
index 463e329..be043f1 100644
--- a/flatbuffer.gni
+++ b/flatbuffer.gni
@@ -69,7 +69,7 @@
     if (defined(invoker.flatc_out_dir)) {
       out_dir = "$root_gen_dir/" + invoker.flatc_out_dir
     } else {
-      out_dir = target_gen_dir
+      out_dir = "$target_gen_dir/snapshot"
     }
 
     outputs = [