Update Makefile

Solve make htmldoc error on python3 ubuntu

Makefile

@@ -156,6 +156,7 @@ phpdoc: clean
 htmldoc:
 	python3 -m venv venv/
 	bash -c 'source venv/bin/activate; \
+	pip install wheel; \
 	pip install mkdocs; \
 	mkdocs build --clean'
 	find doc/html/ -type f -exec chmod a-x '{}' \;