czmq: fix build with glibc 2.24

czmq uses readdir_r(), which has been marked as deprecated in glibc
2.24, and czmq uses -Werror which causes the warning to be turned into
an error that aborts the build.

This commit fixes that by removing -Werror. Long-term, we might want
to upgrade czmq to a newer version, since they've removed the use of
readdir_r() in commit 2594d406d8ec6f54e54d7570d7febba10a6906b2. But
getting rid of -Werror is anyway always a good idea in Buildroot.

Fixes:

  http://autobuild.buildroot.net/results/f6e4b8ed8571fdcf30eecae121dd11dfdef50799



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