Generate javadoc for gerrit-plugin-gwtui

This artifact is required for GWT plugins. To make the consumption of
gerrit-plugin-gwtui easier for plugin developers it should be
published to Maven central. For this having javadocs generated is
required.

Change-Id: I10749b83a7f56c26593371d57abc3862fc1bc289
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>