Add net/if_mib.h header for mac test build

(backport <rust-lang#4022>)
(cherry picked from commit 1bcda05)

Author: GuillaumeGomez

libc-test/build.rs

@@ -221,6 +221,7 @@ fn test_apple(target: &str) {
         "net/if.h",
         "net/if_arp.h",
         "net/if_dl.h",
+        "net/if_mib.h",
         "net/if_utun.h",
         "net/if_var.h",
         "net/ndrv.h",