cirrus.yml: Compile FreeBSD with -Werror

Compiler warnings currently go unnoticed in our FreeBSD builds, since
-Werror is only enabled for Linux and MinGW builds by default. So let's
enable them here now, too.

Reviewed-by: Ed Maste <emaste@freebsd.org>
Message-Id: <20200728074405.13118-3-thuth@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>