Add fast_tensor_util.cpp to .gitignore (#13749)

While working on building TensorFlow I noticed that a file
`fast_tensor_util.cpp` is generated:
```sh
ubuntu@ubuntu:~/tensorflow$ git status
On branch master
Your branch is up-to-date with 'origin/master'.
Untracked files:
  (use "git add <file>..." to include in what will be committed)

        tensorflow/python/framework/fast_tensor_util.cpp

nothing added to commit but untracked files present (use "git add" to track)
ubuntu@ubuntu:~/tensorflow$
```

This fix adds `fast_tensor_util.cpp` to .gitignore so that
it will not be added inadvertently when adding commit with `git add -A`.

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