index: fix build on windows
index: fix build on windows

Reported by mattn.

Fixes #40.

google/codesearch,ksmaheshkumar/codesearch,a5b/codesearch,salsifis/codesearch,a5b/codesearch,salsifis/codesearch,junkblocker/codesearch,google/codesearch,StephanieMak/codesearch,ksmaheshkumar/codesearch,junkblocker/codesearch,StephanieMak/codesearch