Clear FLATBUFFERS_GENERATED_INCLUDES before appending to it.
Clear FLATBUFFERS_GENERATED_INCLUDES before appending to it.

When this library is included by other projects that use the
FLATBUFFERS_GENERATED_INCLUDES variable to build up a list of
generated headers, the FLATBUFFERS_GENERATED_INCLUDES variable
grows to contain *all* headers for *all* libraries leading to
order dependent build errors.

Tested:
Verified everything builds on Linux.

Change-Id: I9e8df6a477763729ee3cbcd5f29ab75f21f52a06

google/motive,google/motive