gitignore: don't ignore the debug npm module

On case insensitive platforms, the rule was catching the debug module
under npm and eslint.

See: https://github.com/nodejs/io.js/pull/1899#issuecomment-109475863
PR-URL: https://github.com/nodejs/io.js/pull/1908
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>

.gitignore

@@ -22,6 +22,7 @@ icu_config.gypi
 
 # various stuff that VC++ produces/uses
 Debug/
+!node_modules/debug/
 Release/
 !doc/blog/**
 *.sln