Fix make clean.

Makefile

@@ -30,6 +30,6 @@ build:
 	python setup.py build_ext --inplace
 
 clean:
-	find . -name '*.pyc' -o -name '*.so' -delete
+	find . -name '*.pyc' -delete -o -name '*.so' -delete
 	find . -name __pycache__ -delete
 	rm -rf .coverage .mypy_cache build compliance/reports dist docs/_build htmlcov MANIFEST src/websockets.egg-info