Allow xmlprovider plugin

Author: tvdijen

composer.json

@@ -23,7 +23,8 @@
             "composer/package-versions-deprecated": true,
             "simplesamlphp/composer-module-installer": true,
             "dealerdirect/phpcodesniffer-composer-installer": true,
-            "phpstan/extension-installer": true
+            "phpstan/extension-installer": true,
+            "simplesamlphp/composer-xmlprovider-installer": true
         }
     },
     "autoload": {