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/winconsoleio.c b/Modules/_io/winconsoleio.c
index 94760ac..a83ef37 100644
--- a/Modules/_io/winconsoleio.c
+++ b/Modules/_io/winconsoleio.c
@@ -12,7 +12,7 @@
 
 #ifdef MS_WINDOWS
 
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>
 #endif