Forgot to implement flush as in jenkinsci/jenkins#3703.

Author: jglick

src/main/java/hudson/tasks/_ant/AntConsoleAnnotator.java

@@ -96,6 +96,11 @@ private boolean endsWith(String line, char c) {
         return len>0 && line.charAt(len-1)==c;
     }
 
+    @Override
+    public void flush() throws IOException {
+        out.flush();
+    }
+
     @Override
     public void close() throws IOException {
         super.close();