[gnome-clocks] widgets: Call Gtk.Button.set_child() instead of Gtk.Button.add()

[Bilal Elmoussaoui <bilelmoussaoui src gnome org>]

commit 360806d03fbb00f63ad01b669b4eeedf7d3a70c1
Author: Yetizone <andreii lisita gmail com>
Date:   Sun Mar 7 16:36:36 2021 +0200

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

 src/widgets.vala | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/src/widgets.vala b/src/widgets.vala
index d3049725..d2f9f5ce 100644
--- a/src/widgets.vala
+++ b/src/widgets.vala
@@ -181,7 +181,7 @@ public class AmPmToggleButton : Gtk.Button {
 
         stack.add_child (am_label);
         stack.add_child (pm_label);
-        add (stack);
+        set_child (stack);
 
         clicked.connect (() => {
             choice = choice == AmPm.AM ? AmPm.PM : AmPm.AM;