Add missing header files to GN files (3 of N)

third_party/flatbuffers/BUILD.gn

 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//testing/test.gni")
 import("//third_party/flatbuffers/flatbuffer.gni")
 
 config("flatbuffers_config") {
   include_dirs = [ "src/include" ]
 }
@@ skipping 60 matching lines @@
 test("flatbuffers_unittest") {
   sources = [
     # The following files are not included in :flatbuffers
     # but are listed here because test.cpp tests more than what will
     # get included into Chrome (reflection and generation).
     "src/include/reflection.h",
     "src/include/reflection_generated.h",
 
     # This is the actual test.
     "src/tests/test.cpp",
+    "src/tests/test.h",
   ]
   deps = [
     ":compiler_files",
     ":flatbuffers",
     ":flatbuffers_samplebuffer",
   ]
   data = [
     "src/tests/",
   ]
 }