Add IPV6_UNICAST_HOPS to fuchsia module
Add IPV6_UNICAST_HOPS to fuchsia module

Recent change #925 added constants for IPV6 multicast, but seems
to have inadvertently left off IPV6_UNICAST_HOPS from fuchsia.

Fixes #932

rust-lang/libc,rust-lang/libc,rust-lang-nursery/libc,rust-lang-nursery/libc,rust-lang/libc,rust-lang-nursery/libc