lookingGlass: Set expand of DebugFlags children

Since commit c290a38730eb2a, St.Bin respects the child's
expand properties instead of basing expansion on the
alignment.

Commit b02800f8 updated a number of callers to prepare for the
change, but the "Flags" tab in looking glass was missed.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3060>

js/ui/lookingGlass.js

@@ -1067,7 +1067,7 @@ const DebugFlag = GObject.registerClass({
     GTypeFlags: GObject.TypeFlags.ABSTRACT,
 }, class DebugFlag extends St.Button {
     _init(label) {
-        const box = new St.BoxLayout();
+        const box = new St.BoxLayout({x_expand: true});
 
         const flagLabel = new St.Label({
             text: label,