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/_sqlite/connection.c b/Modules/_sqlite/connection.c
index 87a9f41..958be7d 100644
--- a/Modules/_sqlite/connection.c
+++ b/Modules/_sqlite/connection.c
@@ -23,7 +23,7 @@
 
 #include "cache.h"
 #include "module.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "connection.h"
 #include "statement.h"
 #include "cursor.h"
diff --git a/Modules/_sqlite/microprotocols.c b/Modules/_sqlite/microprotocols.c
index bb0d19f..3b2d7f4 100644
--- a/Modules/_sqlite/microprotocols.c
+++ b/Modules/_sqlite/microprotocols.c
@@ -24,7 +24,6 @@
  */
 
 #include <Python.h>
-#include <structmember.h>
 
 #include "cursor.h"
 #include "microprotocols.h"