gitignore: never ignore debug module

On case-insensitive platorms, the Debug/ rule catches the debug module
under npm and eslint.

PR-URL: https://github.com/nodejs/node/pull/2286
Reviewed-By: Roman Reiss <me@silverwind.io>

.gitignore

@@ -24,7 +24,7 @@ icu_config.gypi
 
 # various stuff that VC++ produces/uses
 Debug/
-!node_modules/debug/
+!**/node_modules/debug/
 Release/
 !doc/blog/**
 *.sln