zmqpp: add proper header include

The patch has been submitted upstream. If it is accepted, the
patch here will not be necessary.

Fixes http://autobuild.buildroot.net/results/425/42590285167f779e27a4e8db9853c7065b9e31c0/


and some others.

Signed-off-by: Alexander Lukichev <alexander.lukichev@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>