notify-desktop: add meta.mainProgram

Add the meta.mainProgram field to notify-desktop as the `lib.getExe` feature to
substitute the name if this field isn't there is deprecated.

pkgs/tools/misc/notify-desktop/default.nix

···
     license = licenses.gpl2;
     platforms = platforms.unix;
     maintainers = with maintainers; [ ylwghst ];
+    mainProgram = "notify-desktop";
   };
 }