package/zeromq: add libbsd optional dependency

libbsd is an optional dependency which is enabled by default since
version 4.3.3 and
https://github.com/zeromq/libzmq/commit/068385c951c0608edec6264d55ba9c4c923acccc

Fixes:
 - http://autobuild.buildroot.org/results/51220b1b82774e8f6f6ed8593c58d2e3c31a1531



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>