[gnome-games] preferences-window: Add a missing .titlebar

[Alexander Mikhaylenko <alexm src gnome org>]

commit 9bb0c22e54e3c39f95e0c2ec35caaee78a0d9aa6
Author: Alexander Mikhaylenko <alexm gnome org>
Date:   Wed May 27 16:15:27 2020 +0500

    preferences-window: Add a missing .titlebar

 data/ui/preferences-window.ui | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/data/ui/preferences-window.ui b/data/ui/preferences-window.ui
index 6b96dccf..694a9320 100644
--- a/data/ui/preferences-window.ui
+++ b/data/ui/preferences-window.ui
@@ -116,6 +116,9 @@
                       <object class="GtkHeaderBar" id="right_header_bar">
                         <property name="visible">True</property>
                         <property name="show-close-button">True</property>
+                        <style>
+                          <class name="titlebar"/>
+                        </style>
                         <child>
                           <object class="GtkButton" id="page_back_button">
                             <property name="visible">False</property>