Adding some missing import in isDisplaySetActive helper

Packages/ohif-viewerbase/client/lib/helpers/isDisplaySetActive.js

@@ -1,6 +1,7 @@
 import { Template } from 'meteor/templating';
 import { Session } from 'meteor/session';
 import { $ } from 'meteor/jquery';
+import { _ } from 'meteor/underscore';
 
 import { OHIF } from 'meteor/ohif:core';
 import 'meteor/ohif:viewerbase';