Allow nnef_tools to be installed with pip & git

[Maaarcocr]

as mentioned in #106 this would be good to have. I think this has been broken again in commit 51085f21d67e0f6800298a032ce766c61df9cbe0

the error I get is:

  File "setup.py", line 19, in <module>
    shutil.rmtree('build')
  File "/usr/lib/python3.7/shutil.py", line 485, in rmtree
    onerror(os.lstat, path, sys.exc_info())
  File "/usr/lib/python3.7/shutil.py", line 483, in rmtree
    orig_st = os.lstat(path)
FileNotFoundError: [Errno 2] No such file or directory: 'build'

[gyenesvi]

I have fixed the setup script, can you check now?