Added missing header decls

In previous pull requests, see
https://github.com/zeromq/czmq/pull/156
https://github.com/zeromq/czmq/pull/155
https://github.com/zeromq/czmq/pull/153
I added some zmsg/zframe functionality.

Unfortunately I forgot to add function declarations to the headers because I assumed they are autogenerated.

This pull request adds these missing header function declarations.