Building the whl fails with Extension not found

If using Extension, you best import it
Fix break by commit: e5778e4

tensorflow/tools/pip_package/setup.py

@@ -35,6 +35,7 @@ import re
 import sys
 
 from setuptools import Command
+from setuptools import Extension
 from setuptools import find_packages
 from setuptools import setup
 from setuptools.command.install import install as InstallCommandBase