`Function` has no callable signature

`Function.prototype` should be a callable property, currently flow complains that there is no "Callable signature".

The following is valid javascript, thus should not have flow errors:
```javascript
/* @flow */
const noop = Function.prototype;
noop();
```

[flow try](https://flowtype.org/try/#0PQKgBAAgZgNg9gdzCYAoVMCmAXMA7OOABzAF4wAxAVzwGNsBLOPAOiICc5suBPIzANyoCxABQBKAUA)