fix(radio-group): changing vertical to protected from private

Author: kdinev

projects/igniteui-angular/radio/src/radio/radio-group/radio-group.directive.ts

@@ -217,7 +217,7 @@ export class IgxRadioGroupDirective implements ControlValueAccessor, OnDestroy,
      * ```
      */
     @HostBinding('class.igx-radio-group--vertical')
-    private vertical = false;
+    protected vertical = false;
 
     /**
      * A css class applied to the component if any of the