Add missing net/if.h and net/if_mib.h items

Author: GuillaumeGomez

libc-test/build.rs

@@ -1810,6 +1810,7 @@ fn test_freebsd(target: &str) {
                 "net/if.h",
                 "net/if_arp.h",
                 "net/if_dl.h",
+                "net/if_mib.h",
                 "net/route.h",
                 "netdb.h",
                 "netinet/ip.h",