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/genericaliasobject.c b/Objects/genericaliasobject.c
index 49f537e..b8ad4d7 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -2,7 +2,7 @@
 
 #include "Python.h"
 #include "pycore_object.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 typedef struct {
     PyObject_HEAD
@@ -118,7 +118,7 @@
 
     _PyUnicodeWriter writer;
     _PyUnicodeWriter_Init(&writer);
-    
+
     if (ga_repr_item(&writer, alias->origin) < 0) {
         goto error;
     }