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/exceptions.c b/Objects/exceptions.c
index cb661f8..db1ff32 100644
--- a/Objects/exceptions.c
+++ b/Objects/exceptions.c
@@ -9,7 +9,7 @@
 #include "pycore_initconfig.h"
 #include "pycore_object.h"
 #include "pycore_pymem.h"
-#include "structmember.h"
+#include "structmember.h"         // PyMemberDef
 #include "osdefs.h"