fix uclibc's type to u32

[kolapapa]

rust-lang/libc#1979 (comment)

[keeperofdakeys]

Hi, apologies for missing this pull request.

If I'm reading this right glibc (and other libc variants) have getnameinfo's flags parameter as an int, but ulibc is using a uint? I see you've already fixed this upstream, thanks.

Would it be possible to add uclibc to the CI for dns-lookup? Right now I'm using travis to test linux, mac os and freebsd (with appveyor for windows). I assume there is a alternate OS or host triple to use for uclibc? I'll probably move this all to github actions at the same time.