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/complexobject.c b/Objects/complexobject.c
index 8d1461b..a490377 100644
--- a/Objects/complexobject.c
+++ b/Objects/complexobject.c
@@ -6,7 +6,7 @@
 /* Submitted by Jim Hugunin */
 
 #include "Python.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 
 /*[clinic input]
 class complex "PyComplexObject *" "&PyComplex_Type"