Merge pull request #4225 from IgniteUI/dpetev/select-aot

#4208 update IgxSelect leftover item group barrel import (AoT build)

Author: bazal4o

projects/igniteui-angular/src/lib/select/select-group.component.ts

@@ -1,5 +1,5 @@
 import { Component } from '@angular/core';
-import { IgxDropDownGroupComponent } from '../drop-down';
+import { IgxDropDownGroupComponent } from '../drop-down/index';
 
 /**
  * The `<igx-select-item>` is a container intended for row items in