make gerrit site logo (upper left corner) link to site root

like @ gerrit.wikimedia.org

( https://gerrit.wikimedia.org/r/gitweb?p=operations/puppet.git;a=blob;f=files/gerrit/skin/GerritSiteHeader.html;hb=HEAD )

Change-Id: I1bcab2d083a689ba0557690b0f4925dac89b0b16
Reviewed-on: https://review.openstack.org/25632
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins