Fix hidden on host

host selector needs `()` when use as a CSS pseudo-class function

Change-Id: Ic30e3638ab0bc7fe872607d6d1fb09b265de156a
diff --git a/polygerrit-ui/app/styles/shared-styles.ts b/polygerrit-ui/app/styles/shared-styles.ts
index 3b738a4..fbc62e2 100644
--- a/polygerrit-ui/app/styles/shared-styles.ts
+++ b/polygerrit-ui/app/styles/shared-styles.ts
@@ -135,7 +135,7 @@
 
       /* Stopgap solution until we remove hidden$ attributes. */
 
-      :host[hidden],
+      :host([hidden]),
       [hidden] {
         display: none !important;
       }