netfilter: nfnetlink_osf: fix module autoload

Move the alias from xt_osf to nfnetlink_osf.

Fixes: f9324952

 ("netfilter: nfnetlink_osf: extract nfnetlink_subsystem code from xt_osf.c")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>