Auto merge of rust-lang#2461 - rtzoeller:dfly_headers, r=JohnTitor

Fix most libc-test compile warnings and errors for DragonFly

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
```

libc-test/build.rs

@@ -1176,8 +1176,10 @@ fn test_dragonflybsd(target: &str) {
         "ifaddrs.h",
         "langinfo.h",
         "limits.h",
+        "link.h",
         "locale.h",
         "mqueue.h",
+        "net/bpf.h",
         "net/if.h",
         "net/if_arp.h",
         "net/if_dl.h",
@@ -1206,8 +1208,10 @@ fn test_dragonflybsd(target: &str) {
         "sys/ioctl.h",
         "sys/ipc.h",
         "sys/ktrace.h",
+        "sys/malloc.h",
         "sys/mman.h",
         "sys/mount.h",
+        "sys/procctl.h",
         "sys/ptrace.h",
         "sys/resource.h",
         "sys/rtprio.h",
@@ -1219,6 +1223,7 @@ fn test_dragonflybsd(target: &str) {
         "sys/sysctl.h",
         "sys/time.h",
         "sys/times.h",
+        "sys/timex.h",
         "sys/types.h",
         "sys/uio.h",
         "sys/un.h",
@@ -1251,6 +1256,9 @@ fn test_dragonflybsd(target: &str) {
 
             t if t.ends_with("_t") => t.to_string(),
 
+            // sigval is a struct in Rust, but a union in C:
+            "sigval" => format!("union sigval"),
+
             // put `struct` in front of all structs:.
             t if is_struct => format!("struct {}", t),
 
@@ -1284,9 +1292,6 @@ fn test_dragonflybsd(target: &str) {
 
     cfg.skip_struct(move |ty| {
         match ty {
-            // This is actually a union, not a struct
-            "sigval" => true,
-
             // FIXME: These are tested as part of the linux_fcntl tests since
             // there are header conflicts when including them with all the other
             // structs.