[flatbuffers] Make build system pass gn check

There were some broken dependencies that gn check reported
errors for which were easily fixed by moving relevant headers
to :flatbuffers. The other targets depend on :flatbuffers so
they can still use those headers.

Bug: 898837
Change-Id: I259a7e5d781ea17e667b6e6469a1133e875ec7b8
Reviewed-on: https://chromium-review.googlesource.com/c/1299137
Reviewed-by: Balazs Engedy <engedy@chromium.org>
Commit-Queue: Daniel Bratell <bratell@opera.com>
Cr-Commit-Position: refs/heads/master@{#602753}
1 file changed