Add FIO_HAVE_FS_STAT/get_fs_free_size() support for OpenBSD

Add FIO_HAVE_FS_STAT/get_fs_free_size() support for OpenBSD

This commit is OpenBSD version of 60f840a8 in 2015.
This is the same as 60f840a8, as well as the previous commit
for NetBSD, as it uses POSIX statvfs.

Note that fio doesn't compile on OpenBSD 5.9 (hits lex error),
but this this commit should still be good since this function
alone compiles on OpenBSD.

(Note that the function was renamed with _free in c08ad04c)

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>