Generates includes for imported flatbuffers schemas.
Generates includes for imported flatbuffers schemas.

Because includes are generated without directory, this exports the
directory where generated headers are produced to the include path of
the dependent target.

Change-Id: I8058553f69aadf29cc1a2926fc27664d023e6e1d

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