[gnome-clocks] widgets: Call Gtk.Stack.add_child() instead of Gtk.Stack.add()

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit d4a206343b4468a354da7fe273990f0450739e6b
Author: Yetizone <andreii lisita gmail com>
Date:   Sun Mar 7 16:33:58 2021 +0200

    widgets: Call Gtk.Stack.add_child() instead of Gtk.Stack.add()
    
    Gtk.Stack.add() is no longer available in GTK4

 src/widgets.vala | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/widgets.vala b/src/widgets.vala
index a071f2dc..d3049725 100644
--- a/src/widgets.vala
+++ b/src/widgets.vala
@@ -179,8 +179,8 @@ public class AmPmToggleButton : Gtk.Button {
         str = (new GLib.DateTime.utc (1, 1, 1, 12, 0, 0)).format ("%p");
         pm_label = new Gtk.Label (str);
 
-        stack.add (am_label);
-        stack.add (pm_label);
+        stack.add_child (am_label);
+        stack.add_child (pm_label);
         add (stack);
 
         clicked.connect (() => {