`Macro`: Import generic class `stdClass`

Generic empty class `stdClass` is imported as it is one of the return type for `Macro::resolveMacro()`

modules/monitoring/library/Monitoring/Object/Macro.php

@@ -5,6 +5,7 @@ namespace Icinga\Module\Monitoring\Object;
 
 use Exception;
 use Icinga\Application\Logger;
+use stdClass;
 
 /**
  * Expand macros in string in the context of MonitoredObjects