tmux: Fix a problem with displaying Korean letters

tmux have wrong character width with Korean letter used on Mac OS X.

It is already submitted to fix on tmux repository. (http://sourceforge.net/p/tmux/tickets/41/)

It only occur with Mac. I wish this patch is very useful who use Korean.

See also. https://github.com/mxcl/homebrew/pull/16180#issuecomment-16101471