build: simplify xmlFreeNode usage

Noticed while reviewing the previous patch; thankfully, there
are no violations.

* cfg.mk (useless_free_options): Add xmlFreeNode.

cfg.mk

@@ -171,6 +171,7 @@ useless_free_options =				\
   --name=xmlBufferFree				\
   --name=xmlFree				\
   --name=xmlFreeDoc				\
+  --name=xmlFreeNode				\
   --name=xmlXPathFreeContext			\
   --name=xmlXPathFreeObject