gtkcssprovider: deprecate gtk_css_provider_get_default

This function is unused since cdc6e827 commit and does not
contain fallback style.

#1226

gtk/gtkcssprovider.c

@@ -1972,6 +1972,8 @@ gtk_css_provider_load_from_resource (GtkCssProvider *css_provider,
  *
  * Returns: (transfer none): The provider used for fallback styling.
  *          This memory is owned by GTK+, and you must not free it.
+ *
+ * Deprecated: 3.24: Use gtk_css_provider_new() instead.
  **/
 GtkCssProvider *
 gtk_css_provider_get_default (void)

gtk/gtkcssprovider.h

@@ -112,7 +112,7 @@ GDK_AVAILABLE_IN_3_16
 void             gtk_css_provider_load_from_resource (GtkCssProvider *css_provider,
                                                       const gchar    *resource_path);
 
-GDK_AVAILABLE_IN_ALL
+GDK_DEPRECATED_FOR(gtk_css_provider_new)
 GtkCssProvider * gtk_css_provider_get_default (void);
 
 GDK_AVAILABLE_IN_ALL