Fix CMSG_NXTHDR for OSX.
Fix CMSG_NXTHDR for OSX.

This was an oversight from PR #1212.  It's been revealed by the new cmsg
test.

rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang/libc