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/Python/Python-ast.c b/Python/Python-ast.c
index 488276a..80f9164 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -4,7 +4,7 @@
 
 #include "Python.h"
 #include "Python-ast.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 typedef struct {
     int initialized;