vlc: update opencv disabling flag

This patch completes the commit d7f13e50
turning --{with,without}- options in --{enable,disable}- options.

Fixes:
  http://autobuild.buildroot.net/results/a34/a34abdcc92a6b2cf976760d66adf6c7572743bb4/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>