Issue #21124: Fix building _struct on Cygwin.

Patch by Masayuki Yamamoto.
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 796d168..9d48691 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -1650,7 +1650,7 @@
 }
 
 static PyTypeObject unpackiter_type = {
-    PyVarObject_HEAD_INIT(&PyType_Type, 0)
+    PyVarObject_HEAD_INIT(NULL, 0)
     "unpack_iterator",                          /* tp_name */
     sizeof(unpackiterobject),                   /* tp_basicsize */
     0,                                          /* tp_itemsize */