Add missing import to utility python script

tools/clean_output_directory.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 #
 
 import shutil
 import sys
+import subprocess
 import utils
 
 def Main():
   build_root = utils.GetBuildRoot(utils.GuessOS())
   print 'Deleting %s' % build_root
   if sys.platform != 'win32':
     shutil.rmtree(build_root, ignore_errors=True)
   else:
     # Intentionally ignore return value since a directory might be in use.
     subprocess.call(['rmdir', '/Q', '/S', build_root],
                     env=os.environ.copy(),
                     shell=True)
   return 0
 
 if __name__ == '__main__':
   sys.exit(Main())