Prevent constant redefinition when running makepot

Installed as a dependency in my plugin, running makepot.php works but pops a couple of Notices:

```
Notice: Constant PO_MAX_LINE_LEN already defined in .../wp-content/plugins/prompt/wordpress/wp-includes/pomo/po.php on line 12
```

This seems to fix that.
Merge pull request #2 from vernal-creative/require-once

Prevent constant redefinition when running makepot