chance: Avoid abuse of O_NONBLOCK for Windows

O_NONBLOCK from src/lib/w32/w32misc.h is stricly for use with our
fcntl().  pick_seed() passes it to open(), which is wrong.  With any
luck, open() fails as it should anyway (the files being opened are not
expected to exist under Windows).  Messed up in commit 9102ecce5 "Fix
PRNG seeding to resist guessing", v4.3.31.

Clean up: guard with #ifndef _WIN32.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>