Add indetion to avoid TypeError exception thrown in line 322

When I try to generate all the fonts into PNG using command:

```shell
./font-awesome-to-png.py ALL
```

An error occurred like this:

```python
Traceback (most recent call last):
  File "./font-awesome-to-png.py", line 394, in <module>
    export_icon(icon, size, filename, font, color)
  File "./font-awesome-to-png.py", line 322, in export_icon
    borderw = int((size - (bbox[2] - bbox[0])) / 2)
```

I just simply add some indention to that line to avoid this exception