[libhandy] preferences-group: Use the heading style class

[Alexander Mikhaylenko <alexm src gnome org>]

commit bb4f7a781964b5c86b5a2f3fac8e512567958211
Author: Adrien Plazas <kekun plazas laposte net>
Date:   Mon May 25 16:46:15 2020 +0200

    preferences-group: Use the heading style class
    
    This style class has been added to Adwaita in GTK 3.24.14, and matches
    what we want to achieve here.

 src/hdy-preferences-group.ui | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/src/hdy-preferences-group.ui b/src/hdy-preferences-group.ui
index f354a481..be433bdf 100644
--- a/src/hdy-preferences-group.ui
+++ b/src/hdy-preferences-group.ui
@@ -14,6 +14,9 @@
             <property name="halign">start</property>
             <property name="xalign">0</property>
             <style>
+              <!-- Requires Adwaita from GTK 3.24.14. -->
+              <class name="heading"/>
+              <!-- Matching elementary class. -->
               <class name="h4"/>
             </style>
           </object>