Add plugin repositories section in the pom
Add plugin repositories section in the pom

Without this section in the pom, the Eclipse build of gerrit-gwtexpui fails:

Project build error: Unresolveable build extension: Plugin com.googlesource.gerrit:gs-maven-
 wagon:3.3 or one of its dependencies could not be resolved: Failure to find
 com.googlesource.gerrit:gs-maven-wagon:jar:3.3 in http://repo.maven.apache.org/maven2
 was cached in the local repository, resolution will not be reattempted until the update interval
 of central has elapsed or updates are forced

Change-Id: I0fa4b97ae653fbe757ebcfbbab153f22d87b07bb
(cherry picked from commit 475fc60b87a9cb741ebcf8cb8e16ec41a5fc05e1)

basilgor/gerrit,basilgor/gerrit,basilgor/gerrit,basilgor/gerrit,basilgor/gerrit