Modify app/BUILD: bower_components to fetch map files too

In preparation for Polymer 2.x update, as with the webcomponents update,
it pulls in a .map file.

Change-Id: I8c12afbd98780ead07089a6c38e4b7c223d87ea1
diff --git a/polygerrit-ui/app/BUILD b/polygerrit-ui/app/BUILD
index bba0640..19bdcee 100644
--- a/polygerrit-ui/app/BUILD
+++ b/polygerrit-ui/app/BUILD
@@ -51,6 +51,7 @@
         [
             "bower_components/**/*.html",
             "bower_components/**/*.js",
+            "bower_components/**/*.map",
         ],
     ),
 )