Add stackdump to Windows.gitignore

Reasons for making this change:

Ignore the stack dump generated when a program has crashed under Windows.

Links to documentation supporting these rule changes:

http://www.drupalonwindows.com/en/blog/git-shell-windows-reports-shexe-has-stopped-working-appcrash
https://community.atlassian.com/t5/SourceTree-questions/Sourcetree-cannot-clone-pull-anymore-sh-exe-stackdump/qaq-p/82962