kbibtex: specify meta.platforms

This makes Hydra build it on aarch64-linux too.

pkgs/applications/office/kbibtex/default.nix

···
     changelog = "https://invent.kde.org/office/kbibtex/-/raw/v${version}/ChangeLog";
     license = licenses.gpl2Plus;
     maintainers = with maintainers; [ dotlambda ];
+    platforms = platforms.linux;
   };
 }