Issue #1566: HideUtilityClassConstructor violations fixed

src/main/java/com/puppycrawl/tools/checkstyle/gui/Main.java

@@ -36,6 +36,13 @@ public final class Main {
      */
     private static JFrame frame;
 
+    /**
+     * Hidden constructor of the current utility class.
+     */
+    private Main() {
+        // no code
+    }
+
     /**
      * Entry point.
      * @param args the command line arguments.