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-*

.gitignore

@@ -28,6 +28,8 @@
 /DOCS/man/mpv.toc
 
 /waf
+/waf-*
+/waf3-*
 /build
 /.waf*
 /.lock-waf_*