Add execute permission to import_pb_to_tensorboard.py

I used `chmod +x` to add the execute permission.

TESTING

Before this change, you get a `Permission denied` error when trying to run import_pb_to_tensorboard.py:
```
$ ./tensorflow/python/tools/import_pb_to_tensorboard.py
-bash: ./tensorflow/python/tools/import_pb_to_tensorboard.py: Permission denied
```
Tested on Mac OSX Sierra 10.12.6.