windows: annotate libffi as a dependency of _ctypes

Otherwise this isn't captured in the metadata and the library isn't
copied/linked as necessary.

Author: indygreg

cpython-windows/build.py

@@ -44,7 +44,7 @@
         "allow_missing_preprocessor": True
     },
     "_bz2": {},
-    "_ctypes": {},
+    "_ctypes": {"shared_depends": ["libffi-7"]},
     "_decimal": {},
     "_elementtree": {},
     "_hashlib": {