textproc/rubygem-libxml-ruby: Fix build after 8697e0292acea3afe4b1ffaf3741fbebb0002b60 (libmxl2 2.10.1 update)

Reference: http://package19.nyi.freebsd.org/data/123amd64-default-build-as-user/8670a697fb3c/logs/rubygem-libxml-ruby-3.2.3.log