opencv: need threads support

Because the opencv_core module needs threads support, just globally
disable the whole opencv package if the toolchain does not offer this
support.

Fixes:
  http://autobuild.buildroot.org/results/8dd/8dd1674674018a931ba09cc5b414c32360e51692/build-end.log



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