fix: declare property headerSelector (#7495)

The property was created dynamically, it's a more clean approach to
declare it as a member of the class.

samples/client/petstore-security-test/php/SwaggerClient-php/lib/Api/FakeApi.php

@@ -59,6 +59,11 @@ class FakeApi
      */
     protected $config;
 
+    /**
+     * @var HeaderSelector
+     */
+    protected $headerSelector;
+
     /**
      * @param ClientInterface $client
      * @param Configuration   $config