netfilter: nf_osf: move nf_osf_fingers to non-uapi header file

All warnings (new ones prefixed by >>):

>> ./usr/include/linux/netfilter/nf_osf.h:73: userspace cannot reference function or variable defined in the kernel

Fixes: f9324952 ("netfilter: nfnetlink_osf: extract nfnetlink_subsystem code from xt_osf.c")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Fernando Fernandez Mancera <ffmancera@riseup.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>