nginz: Keep /usr/include/zauth.h compatible with other projects

nginz needs libzauth with installs [this file](https://github.com/wireapp/wire-server/blob/master/libs/libzauth/libzauth-c/src/zauth.h) into _any place you want_.

Nevertheless it would great to have only one _/usr/include/zauth.h_ file which should be compatible with zeromq's [file at the same place](https://github.com/zeromq/czmq/blob/master/include/zauth.h)