ui: gtk: Fix a runtime warning on vte >= 0.37

inner-border was dropped in vte API 2.91, in favor of the standard
padding style

Signed-off-by: Cole Robinson <crobinso@redhat.com>
Message-id: 60a6cdc337d611d902f53907e66a8f37ea374d65.1462557436.git.crobinso@redhat.com

[ kraxel: Fix warning with old vte version. ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>