netbsd add deprecation warning for ELAST

Requested in #3040

Author: 0-wiz-0

src/unix/bsd/netbsdlike/netbsd/mod.rs

@@ -1423,6 +1423,12 @@ pub const ENOLINK: ::c_int = 95;
 pub const EPROTO: ::c_int = 96;
 pub const EOWNERDEAD: ::c_int = 97;
 pub const ENOTRECOVERABLE: ::c_int = 98;
+#[deprecated(
+    since = "0.2.143",
+    note = "This value will always match the highest defined error number \
+            and thus is not stable. \
+            See #3040 for more info."
+)]
 pub const ELAST: ::c_int = 98;
 
 pub const F_DUPFD_CLOEXEC: ::c_int = 12;