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/Objects/namespaceobject.c b/Objects/namespaceobject.c
index a28b9e5..29141a8 100644
--- a/Objects/namespaceobject.c
+++ b/Objects/namespaceobject.c
@@ -1,7 +1,7 @@
 // namespace object implementation
 
 #include "Python.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 
 typedef struct {