Plugins with GWT modules need both:
* gerrit-plugin-gwtui
* gerrit-gwtui-common
Export gerrit-gwtui-common in gwtui-api-lib so that only one dependency
is needed to link against in tree plugin build.
This is the same behaviour as when the plugin is built against published
plugin API (we don't publish gwtui-common as standalone artifact).
Change-Id: If8da90dc1038b6d4145d2e2bab333c2642e3c25f