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/sha256module.c b/Modules/sha256module.c
index eee582c..7310826 100644
--- a/Modules/sha256module.c
+++ b/Modules/sha256module.c
@@ -17,7 +17,7 @@
 /* SHA objects */
 
 #include "Python.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "hashlib.h"
 #include "pystrhex.h"