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/methodobject.c b/Objects/methodobject.c
index 4b4927d..f483671 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -2,12 +2,12 @@
 /* Method object implementation */
 
 #include "Python.h"
-#include "pycore_ceval.h"     // _Py_EnterRecursiveCall()
+#include "pycore_ceval.h"         // _Py_EnterRecursiveCall()
 #include "pycore_object.h"
 #include "pycore_pyerrors.h"
 #include "pycore_pymem.h"
-#include "pycore_pystate.h"   // _PyThreadState_GET()
-#include "structmember.h"
+#include "pycore_pystate.h"       // _PyThreadState_GET()
+#include "structmember.h"         // PyMemberDef
 
 /* undefine macro trampoline to PyCFunction_NewEx */
 #undef PyCFunction_New