ibus: add meta.mainProgram (#423754)

pkgs/tools/inputmethods/ibus/default.nix

···
     description = "Intelligent Input Bus, input method framework";
     license = lib.licenses.lgpl21Plus;
     platforms = lib.platforms.linux;
+    mainProgram = "ibus";
     maintainers = with lib.maintainers; [ ttuegel ];
   };
 })