win32.h: revert r37337

* include/ruby/win32.h (fstat): revert r37337, which uses _fstati64()
  instead of fstati64() on mingw32.  [Bug #7276]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38992 b2dd03c8-39d4-4d8f-98ff-823fe69b080e