Avoid implicit conversion from float to double. (#7003)

Avoid implicit conversion from float to double. (#7003)

https://github.com/tensorflow/tflite-micro makes use of flatbuffers with
a variety of DSP toolchains.

Without the change from this PR, we can get a double-promotion warning
with some of these DSP toolchains:
```
flatbuffers/include/flatbuffers/util.h:104:11: error: implicit conversion increases floating-point precision: 'std::numeric_limits<float>::_Ty' (aka 'float') to 'double' [-Werror,-Wdouble-promotion]
  T eps = std::numeric_limits<float>::epsilon();
    ~~~   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```