libvncserver: disable libva support

The libvncclient library (part of libvncserver) has optional support
for HW accelerated H264 video playback using libva

However there are currently two problems with it:

1) it only supports libva compiled with X11 support enabled, but it
   does not verify that is the case, resulting in compile errors.

2) it only supports the libva 1.0.x API, and not newer
   libva versions, like 1.4.x we include with buildroot.

   https://github.com/LibVNC/libvncserver/issues/11



Disabling libva support for now.

Signed-off-by: Floris Bos <bos@je-eigen-domein.nl>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>