bpo-40268: Remove unused structmember.h includes (GH-19530)

If only offsetof() is needed: include stddef.h instead.

When structmember.h is used, add a comment explaining that
PyMemberDef is used.
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index 924ffb5..a8e55c3 100644
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -11,7 +11,7 @@
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 #include "pycore_object.h"
-#include "structmember.h"
+#include <stddef.h>               // offsetof()
 #include "_iomodule.h"
 
 /*[clinic input]