Bazel: fix build

The Bazel build was fixed in I3e9673b95 but the breakage was
accidentally reintroduced in: Ia88f6ff5d.

Change-Id: I734674850043cf0c7aed45bc978cb369846c1fb9
diff --git a/tools/bzl/gwt.bzl b/tools/bzl/gwt.bzl
index 27e0740..7eff506 100644
--- a/tools/bzl/gwt.bzl
+++ b/tools/bzl/gwt.bzl
@@ -231,7 +231,7 @@
       'unzip -qo $$ROOT/$(location :%s);' % opt +
       'mkdir -p $$(dirname $@);' +
       'zip -qr $$ROOT/$@ .',
-    out = 'ui_optdbg' + suffix + '.zip',
+    outs = ['ui_optdbg' + suffix + '.zip'],
     visibility = ['//visibility:public'],
    )