bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS. (GH-6030)

METH_NOARGS functions need only a single argument but they are cast
into a PyCFunction, which takes two arguments.  This triggers an
invalid function cast warning in gcc8 due to the argument mismatch.
Fix this by adding a dummy unused argument.

Modules/_struct.c

@@ -1629,7 +1629,7 @@ unpackiter_traverse(unpackiterobject *self, visitproc visit, void *arg)
 }
 
 static PyObject *
-unpackiter_len(unpackiterobject *self)
+unpackiter_len(unpackiterobject *self, PyObject *Py_UNUSED(ignored))
 {
     Py_ssize_t len;
     if (self->so == NULL)