Composer: clarify that the PHP libxml extension is required

The `simplexml` and `xmlwriter` extensions both need the `libxml` extension, so the requirement not being explicit was not problematic, as it was already enforced via the other two requirements.

Still, better to be explicit.

Author: jrfnl

composer.json

@@ -35,6 +35,7 @@
     },
     "require": {
         "php": ">=7.2.0",
+        "ext-libxml": "*",
         "ext-simplexml": "*",
         "ext-tokenizer": "*",
         "ext-xmlwriter": "*"