Unbreak build with libxml >=2.12.0

Index: src/utils/xml/libxml.cpp
--- src/utils/xml/libxml.cpp.orig
+++ src/utils/xml/libxml.cpp
@@ -36,6 +36,8 @@
 
 #include <fstream>
 
+#include <libxml/parser.h>
+
 #include "debug.h"
 
 namespace
