udp: provide udp{4,6}_lib_lookup for nf_socket_ipv{4,6}

Since commit ca065d0c ("udp: no longer use SLAB_DESTROY_BY_RCU")
the udp6_lib_lookup and udp4_lib_lookup functions are only
provided when it is actually possible to call them.

However, moving the callers now caused a link error:

net/built-in.o: In function `nf_sk_lookup_slow_v6':
(.text+0x131a39): undefined reference to `udp6_lib_lookup'
net/ipv4/netfilter/nf_socket_ipv4.o: In function `nf_sk_lookup_slow_v4':
nf_socket_ipv4.c:(.text.nf_sk_lookup_slow_v4+0x114): undefined reference to `udp4_lib_lookup'

This extends the #ifdef so we also provide the functions when
CONFIG_NF_SOCKET_IPV4 or CONFIG_NF_SOCKET_IPV6, respectively
are set.

Fixes: 8db4c5be ("netfilter: move socket lookup infrastructure to nf_socket_ipv{4,6}.c")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>