make rust-zmq build with nightly


try to compile with today's nightly rust package and was failing with (oct 13th )

> rc/zmq/lib.rs:30:13: 30:33 error: expected constant expr for vector length: non-constant path in constant expr src/zmq/lib.rs:30 type Msg_ = [c_char, ..MsgSize_];
                                                     ^~~~~~~~~~~~~~~~~~~~

Not sure why, I am a noob in rust, but the guide for nightly recommands using consts instead of static when needing non mutable globals.