git-gui/gitk: Do not use a Cygwin-specific kill flag on Windows

I'm posting this here instead of upstream to get some early feedback from Windows people, and maybe to carry this patch around for a while for testing before submitting it upstream.

Merging this would allow us to get rid of @angavrilov's [patch](https://github.com/msysgit/msysgit/blob/master/src/rt/patches/0004-Added-Cygwin-implementation-of-kill.patch) which makes us ship Cygwin's instead of MSYS' `kill`.