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/fileio.c b/Modules/_io/fileio.c
index 1855b83..caf91df 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -3,7 +3,7 @@
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
 #include "pycore_object.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include <stdbool.h>
 #ifdef HAVE_SYS_TYPES_H
 #include <sys/types.h>