package/iprutils: needs zlib

Dependency was introduced by
https://sourceforge.net/p/iprdd/iprutils/ci/1f24b814e56939fd09a2319323e604594784fef3/

Fixes
http://autobuild.buildroot.net/results/48e/48e1c7fb920cbc81eee1cf2f43d1551a0a9ce31e/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>