Add conditional compile attr to `is_ingress`

Closes: #216
Approved by: whitequark

src/socket/mod.rs

@@ -67,6 +67,7 @@ pub(crate) enum PollAt {
 }
 
 impl PollAt {
+    #[cfg(feature = "socket-tcp")]
     fn is_ingress(&self) -> bool {
         match self {
             &PollAt::Ingress => true,