spice-app: fix running when !CONFIG_OPENGL

Do not set 'gl' parameter, fixes:
qemu-system-x86_64: Invalid parameter 'gl'

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20190524130946.31736-7-marcandre.lureau@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>