Add missing ReceiverModelHandler to joran linker

Without the handler the joran default processor is not able to
instanciate a ServerSocketReceiver.

Author: Michael Albiez

logback-classic/src/main/java/ch/qos/logback/classic/joran/ModelClassToModelHandlerLinker.java

@@ -61,6 +61,7 @@ public void link(DefaultProcessor defaultProcessor) {
         defaultProcessor.addHandler(RootLoggerModel.class, RootLoggerModelHandler::makeInstance);
         defaultProcessor.addHandler(LoggerModel.class, LoggerModelHandler::makeInstance);
         defaultProcessor.addHandler(LevelModel.class, LevelModelHandler::makeInstance);
+        defaultProcessor.addHandler(ReceiverModel.class, ReceiverModelHandler::makeInstance);
 
         defaultProcessor.addAnalyser(LoggerModel.class,
                 () -> new RefContainerDependencyAnalyser(context, LoggerModel.class));