fix incorrectly signed ipi6_ifindex in notbsd

Author: mcginty

src/unix/notbsd/mod.rs

@@ -186,7 +186,7 @@ s! {
 
     pub struct in6_pktinfo {
         pub ipi6_addr: ::in6_addr,
-        pub ipi6_ifindex: ::c_uint,
+        pub ipi6_ifindex: ::c_int,
     }
 }