opencv: add zlib dependency

The opencv package selects zlib, but it does not depend on it. This
leads to cases where opencv gets built before zlib, and in this case,
opencv uses its internal version of zlib, which doesn't build properly
(it has some conditional code for ARM that is probably broken), see:

  http://autobuild.buildroot.org/results/45f4e195fe0cc5acace10287f5ff33aa953d5430/build-end.log



We fix this by properly adding the opencv dependency on zlib.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>