netfilter: nfnetlink_osf: add missing enum in nfnetlink_osf uapi header

xt_osf_window_size_options was originally part of
include/uapi/linux/netfilter/xt_osf.h, restore it.

Fixes: bfb15f2a ("netfilter: extract Passive OS fingerprint infrastructure from xt_osf")
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>