Make flatbuffers work with ArduinoSTL (#4357) [C++]

[rstoica]

This fixes #4357 by conditionally including utility over utility.h for ArduinoSTL based upon the include guard of the library's header (no better way to check afaik on which lib is in use).

Legacy compatibility with StandardCplusplus lib (utility.h) is maintained.

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[rstoica]

I signed it!

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[aardappel]

Thanks!