Adds missing Google Flatbuffers license information (#1968)

Author: RussellSpitzer

plugins/spark/v3.5/spark/LICENSE

@@ -375,6 +375,14 @@ License: Apache License 2.0 - https://www.apache.org/licenses/LICENSE-2.0.txt
 
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+This binary artifact contains Google FlatBuffers.
+
+Copyright: 2013-2020 Google Inc.
+Home page: https://google.github.io/flatbuffers/
+License: http://www.apache.org/licenses/LICENSE-2.0
+
+--------------------------------------------------------------------------------
+
 This binary artifact contains ThreeTen.
 
 Copyright: 2007-present, Stephen Colebourne & Michael Nascimento Santos.