floss: Fix Flatbuffer build rework

Since CL:1866813, bt_flatc_bundler should no longer produce a header
file, but produce a c++ compilation target from a generated .cc file.

Bug: 203870024
Tag: #floss
Test: Manual - build floss on Linux

Change-Id: Id774227f36b4deb3f3cebe5b225ec3fb32c13253