net: core: Fix build with CONFIG_IPV6=m

Stephen Rothwell reports the following link failure with IPv6 as module:

  x86_64-linux-gnu-ld: net/core/filter.o: in function `sk_lookup':
  (.text+0x19219): undefined reference to `__udp6_lib_lookup'

Fix the build by only enabling the IPv6 socket lookup if IPv6 support is
compiled into the kernel.

Signed-off-by: Joe Stringer <joe@wand.net.nz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>