zyre: backport fix to build with recent versions of ZeroMQ

Since the recent bump of ZeroMQ, Zyre failed to build, due to the
const qualifier being dropped in a number of functions.

A backport from the upstream Github repository fixes the problem, and
therefore fixes the following build failure:

  http://autobuild.buildroot.org/results/d24/d24d7ccb878151bd81003763ba1a6bcd91d41dc9/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>