sdl: disable x11 in host-sdl

We do not need an X11-capable host SDL, so just disable X11 support.

This currently works if the build machine has the X11 development
packages installed.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>