gitignore: don't ignore debug source directory in V8

On case-insensitive platforms, the Debug/ rule catches it.

PR-URL: https://github.com/nodejs/node/pull/3351
Reviewed-By: indutny - Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>

.gitignore

@@ -25,6 +25,7 @@ icu_config.gypi
 # various stuff that VC++ produces/uses
 Debug/
 !node_modules/debug/
+!deps/v8/src/debug/
 Release/
 !doc/blog/**
 *.sln