games/ufoai: Prepare for mxml 3.1

src/common/xml.cpp:523:14: error: member access into incomplete type 'mxml_node_t' (aka '_mxml_node_s')
type = node->value.element.name;
^
/usr/local/include/mxml.h:97:16: note: forward declaration of '_mxml_node_s'
typedef struct _mxml_node_s mxml_node_t; /** An XML node. **/
^
9 warnings and 1 error generated.

PR: 240423
Approved by: m.tsatsenko@gmail.com (maintainer timeout, 14 days)