Avoid reference to undefined name: stderr does not exist, sys.stderr does

contrib/linearize/linearize-hashes.py

@@ -140,7 +140,7 @@ if __name__ == '__main__':
 	if 'datadir' in settings and not use_userpass:
 		use_datadir = True
 	if not use_userpass and not use_datadir:
-		print("Missing datadir or username and/or password in cfg file", file=stderr)
+		print("Missing datadir or username and/or password in cfg file", file=sys.stderr)
 		sys.exit(1)
 
 	settings['port'] = int(settings['port'])