Don't try to connect to X server from PuTTY

I quite like to leave X forwarding on for PuTTY when connecting to my home
machine, so I can run programs with Xming. There's a two-second delay in Vim
as it tries to connect to the X server whenever I forward it if Xming isn't
running. This fixes it.

http://stackoverflow.com/a/10718757/493111