Add missing const.

--- librsvg-2.40.21/rsvg-css.c	2018-10-01 23:50:22.000000000 +0100
+++ librsvg-2.40.21/rsvg-css.c	2024-05-15 18:42:32.573852578 +0100
@@ -839,7 +839,7 @@
 }
 
 static void
-rsvg_xml_noerror (void *data, xmlErrorPtr error)
+rsvg_xml_noerror (void *data, const xmlError *error)
 {
 }