Doc: define PY_SSIZE_T_CLEAN always (GH-12794)

(cherry picked from commit c88feceb449d6e85d7e17ec36559206094d10d81)

Co-authored-by: Inada Naoki <songofacandy@gmail.com>
diff --git a/Doc/includes/custom2.c b/Doc/includes/custom2.c
index 51ab4b8..6477a19 100644
--- a/Doc/includes/custom2.c
+++ b/Doc/includes/custom2.c
@@ -1,3 +1,4 @@
+#define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include "structmember.h"