docs: Fix version for xfce_panel_set_image_from_source()

The `scale` argument was only added in 4.17.4, so better not to
encourage the use of this function before.

Related: !83, 0a732bdc

docs/reference/libxfce4panel-docs.xml

@@ -139,6 +139,7 @@
   </index>
   <index id="api-index-4-18">
     <title>Index of new symbols in 4.18</title>
+    <xi:include href="xml/api-index-4.17.4.xml"><xi:fallback /></xi:include>
     <xi:include href="xml/api-index-4.17.2.xml"><xi:fallback /></xi:include>
   </index>
   <index id="api-index-4-14">

libxfce4panel/xfce-panel-convenience.c

@@ -328,7 +328,7 @@ xfce_panel_pixbuf_from_source (const gchar  *source,
  *
  * See also: #XfcePanelImage
  *
- * Since: 4.17.2
+ * Since: 4.17.4
  **/
 void
 xfce_panel_set_image_from_source (GtkImage     *image,