ITS#8435 Move sc_writewait to end of slap_callback

So any initializers in code predating sc_writewait
will find sc_private in the expected position.

servers/slapd/slap.h

@@ -2374,8 +2374,8 @@ typedef struct slap_callback {
 	struct slap_callback *sc_next;
 	slap_response *sc_response;
 	slap_response *sc_cleanup;
-	slap_writewait *sc_writewait;
 	void *sc_private;
+	slap_writewait *sc_writewait;
 } slap_callback;
 
 struct slap_overinfo;