gitignore: ignore waf directory on Windows

In native Windows, waflib is kept in /waf-1.8.4-* or /waf3-1.8.4-*
instead of /.waf-1.8.4-* and /.waf3-1.8.4-*
Merge pull request #1314 from rossy/win-waf-gitignore

gitignore: ignore waf directory on Windows