net/ucx: fix build on FreeBSD 13.5 by defining s6_addr32 shim

FreeBSD's struct in6_addr does not provide s6_addr32 directly. Add a small
shim in src/uct/ib/base/ib_device.c to map s6_addr32 to the FreeBSD layout
(u6_addr.u6_addr32) when s6_addr32 is not already available.

PR: 293429
Tested by: kib
MFH: 2026Q1

(cherry picked from commit 63a68d9d459a0ed27f7b1dadaee6cf4cdbe8e2af)