vlc: ensure correct directfb-config is used

Otherwise we can end up using host includes/libraries if the build host has
directfb-config.

Fixes:
http://autobuild.buildroot.net/results/361/3612b54b0bbc2a55ba6f0febb7be32595b523d96/



Signed-off-by: Peter Korsgaard <peter@korsgaard.com>