Fix build error with GCC 7.2.1 on AWS Linux 2 (#16470)

This fix fixes a build failure when compiling with
GCC 7.2.1 on AWS Linux 2:
```
gcc version 7.2.1 20170915 (Red Hat 7.2.1-2) (GCC)
```

The eror output was:
```
...
./tensorflow/contrib/lite/toco/model.h:1567:25: error: 'std::function' has not been declared
   void EraseArrays(std::function<bool(const string&)> discardable) {
  .....
```

This fix is related to 16046.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>