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_libev_stubs.c

@@ -69,7 +69,12 @@ static long hash_loop(value loop) { return (long)Ev_loop_val(loop); }
 
 static struct custom_operations loop_ops = {
     "lwt.libev.loop", custom_finalize_default,  compare_loops,
-    hash_loop,        custom_serialize_default, custom_deserialize_default};
+    hash_loop,        custom_serialize_default, custom_deserialize_default,
+    custom_compare_ext_default,
+#if OCAML_VERSION >= 408000
+    NULL
+#endif
+};
 
 /* Do nothing.
 
@@ -127,7 +132,12 @@ static long hash_watcher(value watcher) { return (long)Ev_io_val(watcher); }
 
 static struct custom_operations watcher_ops = {
     "lwt.libev.watcher", custom_finalize_default,  compare_watchers,
-    hash_watcher,        custom_serialize_default, custom_deserialize_default};
+    hash_watcher,        custom_serialize_default, custom_deserialize_default,
+    custom_compare_ext_default,
+#if OCAML_VERSION >= 408000
+    NULL
+#endif
+};
 
 /* +-----------------------------------------------------------------+
    | IO watchers                                                     |

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)