Restore ability to continue using mlehner/gelf-php in test

Was previously broken by a coding standard commit 392ef35fd4

tests/Monolog/Handler/GelfHandlerLegacyTest.php

@@ -23,6 +23,8 @@ class GelfHandlerLegacyTest extends TestCase
         if (!class_exists('Gelf\MessagePublisher') || !class_exists('Gelf\Message')) {
             $this->markTestSkipped("mlehner/gelf-php not installed");
         }
+
+        require_once __DIR__ . '/GelfMocks.php';
     }
 
     /**