sd-radv: avoid redefinition of struct in6_addr

Both netinet/icmp6.h and linux/in6.h will define struct in6_addr, and in
user space we want to use the netinet/icmp6.h variant.

Fixes build problem:
In file included from src/libsystemd-network/sd-radv.c:23:0:
/home/hegtvedt/work/os/product/sunrise/root/_build/v2/include/linux/in6.h:30:8:
error: redefinition of 'struct in6_addr'