Using `#if WIN32` instead of `#ifdef _WIN32` seems to result in unistd.h being included.

[MarcelPiNacy]

When building in windows, the file file_finder_test.cc will attempt to include unistd.h. It seems this is due to #if WIN32 being used instead of #ifdef _WIN32. I'm not sure if I have my build incorrectly configured, but changing this (with _CRT_SECURE_NO_WARNINGS due to a strncpy call) works.