CyFunction: Fix use of `offsetof()` to silence clang -Wextended-offsetof

Cython/Utility/CythonFunction.c

@@ -414,7 +414,7 @@ static PyGetSetDef __pyx_CyFunction_getsets[] = {
 };
 
 static PyMemberDef __pyx_CyFunction_members[] = {
-    {(char *) "__module__", T_OBJECT, offsetof(__pyx_CyFunctionObject, func.m_module), PY_WRITE_RESTRICTED, 0},
+    {(char *) "__module__", T_OBJECT, offsetof(PyCFunctionObject, m_module), PY_WRITE_RESTRICTED, 0},
     {0, 0, 0,  0, 0}
 };