osdep.h: include sys/types.h for ssize_t definition

sys/types.h is taken out from "ifdef __OpenBSD__" guard. It should be
safe for other systems, according to following survey:
http://hacks.owlfolio.org/header-survey/

This fixes build for CONFIG_IOVEC-less systems (mingw).

Signed-off-by: Igor Mitsyanko <i.mitsyanko@gmail.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
