[gnome-disk-utility] style_context: set css class as a string

[Kai Lüke <kailueke src gnome org>]

commit 6e826b3edb89ab6a0703d9ff2cd4cefe2d59b9db
Author: Maximiliano Sandoval R <msandova gnome org>
Date:   Tue Oct 12 11:46:01 2021 +0200

    style_context: set css class as a string
    
    Those constants are gone in GTK 4.

 src/disks/gduvolumegrid.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/src/disks/gduvolumegrid.c b/src/disks/gduvolumegrid.c
index 2e17f203..91012b40 100644
--- a/src/disks/gduvolumegrid.c
+++ b/src/disks/gduvolumegrid.c
@@ -868,7 +868,7 @@ render_element (GduVolumeGrid *grid,
     {
       guint icon_offset = 0;
       gtk_style_context_save (context);
-      gtk_style_context_add_class (context, GTK_STYLE_CLASS_IMAGE);
+      gtk_style_context_add_class (context, "image");
       for (n = 0; n < icons_to_render->len; n++)
         {
           const gchar *name = icons_to_render->pdata[n];
@@ -912,7 +912,7 @@ render_element (GduVolumeGrid *grid,
   if (element->show_spinner)
     {
       gtk_style_context_save (context);
-      gtk_style_context_add_class (context, GTK_STYLE_CLASS_SPINNER);
+      gtk_style_context_add_class (context, "spinner");
       gtk_render_activity (context, cr,
                            ceil (element->x) + 4,
                            ceil (element->y + element->height - 16 - 4),