gitignore: adding .vs/ directory to .gitignore

This is created by vs 2015 for user & machine-specific files and should
be ignored by git.

PR-URL: nodejs#6070
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

.gitignore

@@ -42,6 +42,7 @@ ipch/
 *.sdf
 *.opensdf
 *.VC.opendb
+.vs/
 
 /config.mk
 /config.gypi