namespace.h: Fix typo: s/CLONE_NEWSNS/CLONE_NEWNS/

This typo causes namespace.h to always unconditionally define
CLONE_NEWNS rather than using the system definition.

Bug present since the initial version of unshare in commit
4205f1fda1ac32046125a2e0f3937b666186bfab.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Anton Cherkashyn <mail@antonc.com>