package/libva-utils: fix build failure when x11 support is disabled

Fixes
http://autobuild.buildroot.net/results/2f8/2f89e41f79e8bec1c0561b486ae5750fc87a6320/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>