Composer: clarify that the PHP libxml extension is required

composer.json

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

requirements.php

@@ -41,6 +41,7 @@ function checkRequirements()
 
     $requiredExtensions = array(
         'tokenizer',
+        'libxml',
         'xmlwriter',
         'SimpleXML',
     );

scripts/BuildRequirementsCheckMatrix.php

@@ -82,7 +82,7 @@ public function getBuilds()
      */
     private function getValidBuilds()
     {
-        $extensions = ['minimal' => 'none, tokenizer, xmlwriter, SimpleXML'];
+        $extensions = ['minimal' => 'none, tokenizer, libxml, xmlwriter, SimpleXML'];
 
         $builds = [];
         foreach ($this->validPhp as $php) {
@@ -145,10 +145,10 @@ private function getInvalidPHPBuilds()
     private function getMissingExtensionsBuilds()
     {
         $extensions = [
-            'missing tokenizer'     => 'none, xmlwriter, SimpleXML',
-            'missing xmlwriter'     => ':xmlwriter',
-            'missing SimpleXML'     => ':SimpleXML',
-            'missing both XML exts' => 'none, tokenizer',
+            'missing tokenizer'    => 'none, xmlwriter, SimpleXML',
+            'missing xmlwriter'    => ':xmlwriter',
+            'missing SimpleXML'    => ':SimpleXML',
+            'missing all XML exts' => 'none, tokenizer',
         ];
 
         $builds = [];