gitlab-ci: Disable vhost-kernel in build-disable job

Commit 299e6f19 ("vhost-net: revamp configure logic") added
the --enable-vhost-kernel option.
Disable it in the build-disable job.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Message-Id: <20210131104621.221602-1-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>