status/system: Mark 'Power Off' menu header for translation

This was missed when the menu was added as part of quick settings.

Luckily we can re-use the msg-id/context from the shutdown dialog,
so the change is not subject to the string freeze.

https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/5862

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2484>

js/ui/status/system.js

@@ -164,7 +164,7 @@ class ShutdownItem extends QuickSettingsItem {
         this._systemActions = new SystemActions.getDefault();
         this._items = [];
 
-        this.menu.setHeader('system-shutdown-symbolic', 'Power Off');
+        this.menu.setHeader('system-shutdown-symbolic', C_('title', 'Power Off'));
 
         this._addSystemAction(_('Suspend'), 'can-suspend', () => {
             this._systemActions.activateSuspend();