vlc: make opencv explicitly configurable

World builds are failing due to change in opencv in API.
Since vlc is only using opencv as an example and a filter, disable by default.
Revisit when upstream adapts to new opencv API.

Signed-off-by: Tim Orling <TicoTimo@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>