Fix build with newer libxml; checked was removed in:
https://gitlab.gnome.org/GNOME/libxml2/-/commit/ce76ebfd1312459951d555ad9d87fb9a89eede55

Index: src/raptor_libxml.c
--- src/raptor_libxml.c.orig
+++ src/raptor_libxml.c
@@ -245,17 +245,6 @@ raptor_libxml_getEntity(void* user_data, const xmlChar
     }
     
     ret->owner = 1;
-
-#if LIBXML_VERSION >= 20627
-    /* Checked field was released in 2.6.27 on 2006-10-25
-     * http://git.gnome.org/browse/libxml2/commit/?id=a37a6ad91a61d168ecc4b29263def3363fff4da6
-     *
-     */
-
-    /* Mark this entity as having been checked - never do this again */
-    if(!ret->checked)
-      ret->checked = 1;
-#endif
   }
 
   return ret;
