Fix complication on Solaris

With the net feature enabled, but with os-poll disabled.
diff --git a/src/sys/shell/tcp.rs b/src/sys/shell/tcp.rs
index a2baaa6..0ed225f 100644
--- a/src/sys/shell/tcp.rs
+++ b/src/sys/shell/tcp.rs
@@ -90,6 +90,7 @@
     target_os = "ios",
     target_os = "freebsd",
     target_os = "netbsd",
+    target_os = "solaris",
 ))]
 pub(crate) fn get_keepalive_time(_: TcpSocket) -> io::Result<Option<Duration>> {
     os_required!()