[ocaml] Restore compatibility with OCaml 4.12

OCaml 4.12 has introduced a new field in the `Gc.stats` record,
`forced_major_collections`. This makes compilation of Dune fail due to
`src/catapult/catapult.ml:fake_gc_stat`

We call `Gc.quick_stats` and override the set of safe fields.

Fixes: ocaml#3583

cc: ocaml#2298

Signed-off-by: Emilio Jesus Gallego Arias <e+git@x80.org>

src/catapult/catapult.ml

@@ -10,7 +10,9 @@ type t =
   }
 
 let fake_gc_stat =
-  { Gc.minor_words = 0.
+  let init_gc = Gc.quick_stat () in
+  { init_gc with
+    Gc.minor_words = 0.
   ; promoted_words = 0.
   ; major_words = 0.
   ; minor_collections = 0
@@ -26,7 +28,7 @@ let fake_gc_stat =
   ; compactions = 0
   ; top_heap_words = 0
   ; stack_size = 0
-  }
+  } [@ocaml.warning "-23"]      (* all fiels of record used *)
 
 let fake time_ref buf =
   let print s = Buffer.add_string buf s in