gitignore: don't ignore the debug npm module

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

See: #1899 (comment)
PR-URL: #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