Doc: define PY_SSIZE_T_CLEAN always (GH-12794)

diff --git a/Doc/includes/custom4.c b/Doc/includes/custom4.c
index 0994d8f..b0b2906 100644
--- a/Doc/includes/custom4.c
+++ b/Doc/includes/custom4.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "structmember.h"