Fix most libc-test compile warnings and errors for DragonFly

[rtzoeller]

libc-test is failing on my DragonFly BSD system with a large number of errors, most of which are just a result of headers not being included.

[root@ ~/libc/libc-test]# uname -a
DragonFly  6.0-RELEASE DragonFly v6.0.1-RELEASE #0: Wed Oct 13 21:06:17 CDT 2021     root@:/usr/obj/usr/src/sys/X86_64_GENERIC  x86_64

[root@ ~/libc/libc-test]# cargo test 2>&1 >/dev/null | wc -l
    6015

Including a few additional headers, most of these warnings and errors are fixed:

[root@ ~/libc/libc-test]# cargo test 2>&1 >/dev/null | wc -l
    561

[rust-highfive]

r? @JohnTitor

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[JohnTitor]

Thanks! Unfortunately, we don't have CI for DragonFlyBSD and it's hard to find currently :(

[JohnTitor]

@bors r+

[bors]

📌 Commit 0c4b6ed has been approved by JohnTitor

[bors]

⌛ Testing commit 0c4b6ed with merge 4aaba6c...

[bors]

☀️ Test successful - checks-actions, checks-cirrus-freebsd-11, checks-cirrus-freebsd-12, checks-cirrus-freebsd-13
Approved by: JohnTitor
Pushing 4aaba6c to master...