Fully initialize custom_operations job_ops

It was missing an initializer for `.compare_ext` (since 3.12.1), the
default is `custom_compare_ext_default`; and `.fixed_length` (since
4.08.0) for which `NULL` is good.

src/unix/lwt_unix_stubs.c

@@ -19,6 +19,7 @@
 #include <caml/memory.h>
 #include <caml/mlvalues.h>
 #include <caml/signals.h>
+#include <caml/version.h>
 
 #include <assert.h>
 #include <errno.h>
@@ -989,7 +990,12 @@ static void *worker_loop(void *data) {
 /* Description of jobs. */
 struct custom_operations job_ops = {
     "lwt.unix.job",      custom_finalize_default,  custom_compare_default,
-    custom_hash_default, custom_serialize_default, custom_deserialize_default};
+    custom_hash_default, custom_serialize_default, custom_deserialize_default,
+    custom_compare_ext_default,
+#if OCAML_VERSION >= 40800
+    NULL
+#endif
+};
 
 /* Get the job structure contained in a custom value. */
 #define Job_val(v) *(lwt_unix_job *)Data_custom_val(v)