Update flatbuffers rules to allow includes.
Update flatbuffers rules to allow includes.

Change-Id: I24dec21ce0fe472cd7cde77e883a9eabbb45b87a

fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers,fuchsia-mirror/third_party-flatbuffers