Try to squelch a GCC warning.

Try to squelch a GCC warning.

Maybe I've gotten lost in a twisty little maze of typedefs and array
pointers, all different, but I don't see how casting a const struct
icmp6_hdr * to a const nd_ipv6 * discards constness, and clang on my Mac
agrees with me.

First cast to const u_char * and then to const nd_ipv6 *, in the hopes
that GCC won't complain.