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/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index 319dc52..8640276 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -6,7 +6,7 @@
 
 #define PY_SSIZE_T_CLEAN
 #include "Python.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "multibytecodec.h"
 #include "clinic/multibytecodec.c.h"