xml: make error struct constant

As of https://gitlab.gnome.org/GNOME/libxml2/-/commit/61034116d0a3c8b295c6137956adc3ae55720711, the error struct in the callback is const

src/arvxmlschema.c

@@ -69,7 +69,7 @@ typedef struct {
 } XmlSchemaError;
 
 static void
-_structured_error_handler (void *ctx, xmlErrorPtr error)
+_structured_error_handler (void *ctx, const xmlError* error)
 {
 	XmlSchemaError *schema_error = ctx;