Fix polygerrit-ui/BUILD to ensure plugins can be tested

Release-Notes: skip
Change-Id: Id7112fef20751452cfb3ee979421279a35bd4214

polygerrit-ui/BUILD

diff --git a/polygerrit-ui/BUILD b/polygerrit-ui/BUILD
index 640b763..2f405e6 100644
--- a/polygerrit-ui/BUILD
+++ b/polygerrit-ui/BUILD
@@ -58,3 +58,6 @@
         "@ui_dev_npm//mocha",
     ],
 )
+
+# This is used by plugins.
+exports_files(["karma.conf.js"])