Fix incremental compilation with ppx's

Same fix as 6a3c51c358 but also for ppx's

src/super_context.ml

@@ -791,7 +791,14 @@ module PP = struct
     add_rule sctx
       (libs
        >>>
-       Build.dyn_paths (Build.arr (Lib.archive_files ~mode ~ext_lib:ctx.ext_lib))
+       Build.dyn_paths (Build.arr (fun libs ->
+         List.rev_append
+           (Lib.archive_files ~mode ~ext_lib:ctx.ext_lib libs)
+           (List.filter_map libs ~f:(function
+              | Lib.Internal (dir, lib) ->
+                Some (Path.relative dir (lib.name ^ ctx.ext_lib))
+              | External _ ->
+                None))))
        >>>
        Build.run ~context:ctx (Ok compiler)
          [ A "-o" ; Target target