zeromq: require IPv6 support in toolchain

The zeromq package was recently bumped from version 2.2.0 to version 3.2.2,
which has introduced a requirement for IPv6 support in the toolchain. At
present, IPv6 support is mandatory in zeromq: there is no configuration option
to enable/disable the feature.

Signed-off-by: Simon Dawson <spdawson@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>