vlc: explicitly disable opencv support

vlc can use opencv in video filters, but it requires a couple of
features enabled in opencv; so, simply force disabling opencv support.

Note that vlc does includes the legacy opencv/cv.h header, which pulls a
number of opencv's features (core, video, imgproc, ...).

Fixes:
  http://autobuild.buildroot.org/results/39c/39c77ffb5a5599d0b09422433c747b2bac185c4f/
  http://autobuild.buildroot.org/results/a79/a79b055da09e4d8ede263251df8461bc8a64569a/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>