spice: Remove unused include

The definitions in the header are not  used.
Also this fixes porting SPICE to Windows where the header is not
available.

Signed-off-by: Frediano Ziglio <fziglio@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190107184404.31993-1-fziglio@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>