setup: get the b2xcompat extension installed

Summary: Now that we have it, we wants it deployable

Test Plan:
ran setup.py install --prefix=/tmp/babar/

The file was there!

Reviewers: #sourcecontrol

Differential Revision: https://phabricator.fb.com/D1990894

setup.py

@@ -12,6 +12,7 @@ setup(
     keywords='fb hg mercurial',
     license='',
     py_modules=[
+        'b2xcompat',
         'backups',
         'chistedit',
         'fbamend',