Exclude GWT library from gerrit_plugin_gwtui artifact

GWT user and dev libraries are shaded into gerrit_plugin_gwtui and
interferes the compilation of GWT plugins in standalone build mode.

The right way to fix that would be to avoid the inclusion of the
GWT libraries in gerrit_plugin_gwtui artifact in the first place.
As quick and dirty workaround strip the GWT libraries during fetch
of the plugin API from Central.

Change-Id: I76762cc1adf101f865ee58ff479a078a73211657
diff --git a/gerrit_api.bzl b/gerrit_api.bzl
index 5c2e893..4b1f09d 100644
--- a/gerrit_api.bzl
+++ b/gerrit_api.bzl
@@ -19,6 +19,7 @@
     name = 'gerrit_plugin_gwtui',
     artifact = 'com.google.gerrit:gerrit-plugin-gwtui:' + VER,
     sha1 = 'ccf9fe33a759c247339ab096ac505013aed27a17',
+    exclude = ['com/google/gwt/*'],
   )
   maven_jar(
     name = 'gerrit_acceptance_framework',