Add obus to native build inputs

Otherwise the interface generator is not found:

     > 11 |   (run obus-gen-interface -keep-common -o systemd_interfaces %{deps})))
     >             ^^^^^^^^^^^^^^^^^^
     > Error: Program obus-gen-interface not found in the tree or in PATH
     >  (context: default)
     > File "bindings/timedate/dune", line 11, characters 7-25:
     > 11 |   (run obus-gen-interface -keep-common -o timedate_interfaces %{deps})))
     >             ^^^^^^^^^^^^^^^^^^
     > Error: Program obus-gen-interface not found in the tree or in PATH
     >  (context: default)

Author: knuton

controller/default.nix

@@ -28,6 +28,7 @@ ocamlPackages.buildDunePackage rec {
 
   nativeBuildInputs = [
     discount # Transform Markdown to HTML
+    ocamlPackages.obus
   ];
 
   buildInputs = with ocamlPackages; [