Recent CI builds are failing because the `offsetof` function in
`Modules/zlibmodule.c` cannot be called. For example, see
https://buildbot.python.org/all/#/builders/15/builds/5217/steps/4/logs/stdio
from commit fabcbe9. Here is a relevant
snippet from the stdio logs for step 4 compile:

```
../Modules/zlibmodule.c:1802:21: error: implicit declaration of function ‘offsetof’ [-Werror=implicit-function-declaration]
 #define COMP_OFF(x) offsetof(compobject, x)
                     ^~~~~~~~
../Modules/zlibmodule.c:1804:39: note: in expansion of macro ‘COMP_OFF’
     {"unused_data",     _Py_T_OBJECT, COMP_OFF(unused_data), Py_READONLY},
                                       ^~~~~~~~
../Modules/zlibmodule.c:1802:21: note: ‘offsetof’ is defined in header ‘<stddef.h>’; did you forget to ‘#include <stddef.h>’?
../Modules/zlibmodule.c:1776:1:
+#include <stddef.h>

../Modules/zlibmodule.c:1802:21:
 #define COMP_OFF(x) offsetof(compobject, x)
                     ^~~~~~~~
../Modules/zlibmodule.c:1804:39: note: in expansion of macro ‘COMP_OFF’
     {"unused_data",     _Py_T_OBJECT, COMP_OFF(unused_data), Py_READONLY},
                                       ^~~~~~~~
../Modules/zlibmodule.c:1802:30: error: expected expression before ‘compobject’
 #define COMP_OFF(x) offsetof(compobject, x)
                              ^~~~~~~~~~
```