xmlmarclint: import fix

Before:
---------
The tool `xmlmarclint` is broken, *for some reason* (as `sys` is indeed imported):
```
$ xmlmarclint /some/filepath
Traceback (most recent call last):
  File "/home/jlavik/envs/pu/bin/xmlmarclint", line 9, in <module>
    load_entry_point('Invenio==1.9999.2.dev20140807133158', 'console_scripts', 'xmlmarclint')()
  File "/home/jlavik/envs/pu/src/invenio/invenio/legacy/bibrecord/scripts/xmlmarclint.py", line 42, in main
    """ % (sys.argv[0])
UnboundLocalError: local variable 'sys' referenced before assignment
```

After:
-------
It works.