Use POSIX path for poll.h and fcntl.h headers

Use POSIX path for poll.h and fcntl.h headers

musl complains when using include paths like <sys/poll.h> rather than
POSIX's <poll.h> so change the poll.h include path and update the
windows poll.h compatibility shim to match.

Allegedly sys/poll.h was needed for GLIBCs that were older than 2.3
according to
https://www.winehq.org/pipermail/wine-patches/2014-June/132877.html but
I can still see that path in an ancient Red Hat 5.0
glibc-devel-2.0.7-32.i386.rpm ...

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>