Adapt to upstream move of MenuItem

In I5b879c4f MenuItem was extracted to its own class.

Change-Id: Ib5ca2dd332ccaaca16b1e30e31c9675dddf75513
diff --git a/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserMenu.java b/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserMenu.java
index f61ea25..859a773 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserMenu.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/serviceuser/ServiceUserMenu.java
@@ -16,6 +16,7 @@
 
 import com.google.common.collect.Lists;
 import com.google.gerrit.extensions.annotations.PluginName;
+import com.google.gerrit.extensions.client.MenuItem;
 import com.google.gerrit.extensions.restapi.AuthException;
 import com.google.gerrit.extensions.webui.TopMenu;
 import com.google.gerrit.server.CurrentUser;