netfilter: nf_nat: fix compiler warning with CONFIG_NF_CT_NETLINK=n

net/ipv4/netfilter/nf_nat_core.c:52: warning: 'nf_nat_proto_find_get' defined but not used
net/ipv4/netfilter/nf_nat_core.c:66: warning: 'nf_nat_proto_put' defined but not used

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>