Add missing extension requirement to composer file

Author: mallardduck

composer.json

@@ -23,6 +23,7 @@
         "php": "^7.3|^7.4|^8.0",
         "ext-dom": "*",
         "ext-json": "*",
+        "ext-libxml": "*",
         "composer-runtime-api": "^2.0",
         "phpunit/phpunit": "^8.3|^9.0",
         "symfony/property-access": "^4.0|^5.0|^6.0",