style: Force symbolic icons in quick settings

The icon for the Settings toggle comes from the .desktop file,
so isn't symbolic by default. Enforce that via the -st-icon-style
property, like the .aggregate-menu style did.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2419>

data/theme/gnome-shell-sass/widgets/_quick-settings.scss

@@ -91,6 +91,8 @@
 .quick-settings-system-item {
   & > StBoxLayout { spacing: 2 * $base_padding; }
 
+  .icon-button > StIcon { -st-icon-style: symbolic; }
+
   & .power-item {
     min-height: 0;
     min-width: 0;