fix ifdef logic in fs_get_free_space()

fix ifdef logic in fs_get_free_space()

- this
#if defined(HAVE_WIN32)
#elif defined(HAVE_SYS_STATVFS_H) || !defined(HAVE_WIN32)
#endif
  is equivalent to
#if defined(HAVE_WIN32)
#else
#endif
  as the HAVE_WIN32 are in the if and the elif
- there is no reason to "care" about "statvfs()" as the fallback is to use
  statfs (see the line above, that I reproduce here):
#ifdef HAVE_SYS_STATVFS_H
#include <sys/statvfs.h>
#else
#define statvfs statfs
#endif
- the other commits related to this piece of code are
c8d10999f1789b88f9f93d29b512e7af10fd009d
33b01b743fa5629828f61fa1300234785c999afa
db82b457bf1dede1225a53c37e60d2d30af84b3b