ui: egl-headless requires dmabuf support

Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20170517122744.3541-1-kraxel@redhat.com