mksysinfo: fix in6_addr in mld_hdr_t for Solaris

Patch by Rainer Orth.

Change-Id: I2b6fda31e46c098ea06c1462310a0a16d304485f
Reviewed-on: https://go-review.googlesource.com/60732
Reviewed-by: Ian Lance Taylor <iant@golang.org>