curl_setup: fix missing `ADDRESS_FAMILY` type in rare build cases

curl_setup: fix missing `ADDRESS_FAMILY` type in rare build cases

Build failed when both `ADDRESS_FAMILY` and `sockaddr_un` stuct were
missing from the Windows SDK, with UnixSockets enabled.

Seen with GNU 4.4.0 in CeGCC 0.59.1:
```
lib/curl_setup.h:983: error: expected specifier-qualifier-list before 'ADDRESS_FAMILY'
lib/curl_setup.h:985: warning: struct has no members
```

Also reported with VS2003:
https://datagirl.xyz/posts/wolfssl_curl_w2k.html

Closes #15969