nbd: skip SIGTERM handler if NBD device support is not built

The termsig_handler function is used by the client thread handling the
host NBD device connection to do a graceful shutdown. IOW, if we have
disabled NBD device support at compile time, we don't need the SIGTERM
handler. This fixes a build issue for Windows.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20200825103850.119911-3-berrange@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>