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/_json.c b/Modules/_json.c
index 1754416..075aa3d 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -9,7 +9,7 @@
 #endif
 
 #include "Python.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "pycore_accu.h"
 
 typedef struct {