Fix GSXML compatibility with libxml2 v2.11.0

Fixes #294.

Author: triplef

Source/Additions/GSXML.m

@@ -2671,10 +2671,12 @@ + (void) initialize
                   return NULL;
                 }
               ret->owner = 1;
+#if LIBXML_VERSION < 21100
               if (ret->checked == 0)
                 {
                   ret->checked = 1;
                 }
+#endif
             }
         }
     }