pantheon.switchboard-plug-power: fix crash

[bobby285271]

Motivation for this change

Similar to #130380 (comment).

(.io.elementary.switchboard-wrapped_:3264): GLib-GIO-ERROR **: 20:51:52.806: Settings schema 'io.elementary.desktop.wingpanel.power' is not installed
[1]    3264 trace trap (core dumped)  io.elementary.switchboard

Haven't found similar issue from other switchboard plugins yet, but would like to fix once reported.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.