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/frameobject.c b/Objects/frameobject.c
index 3c140ac..bdd7862 100644
--- a/Objects/frameobject.c
+++ b/Objects/frameobject.c
@@ -7,7 +7,7 @@
 #include "code.h"
 #include "frameobject.h"
 #include "opcode.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 #define OFF(x) offsetof(PyFrameObject, x)