Add missing dependency

Author: tvdijen

composer.json

@@ -35,6 +35,8 @@
     },
     "require": {
         "php": "^8.1",
+        "ext-libxml": "*",
+
         "simplesamlphp/assert": "^1.1.0",
         "simplesamlphp/composer-module-installer": "^1.3.4",
         "simplesamlphp/simplesamlphp": "^2.2.0",