update import in UI test

Author: joboet

tests/ui/stability-attribute/stability-in-private-module.rs

@@ -1,4 +1,4 @@
 fn main() {
-    let _ = std::sys::os::errno();
+    let _ = std::sys::io::errno();
     //~^ERROR module `sys` is private
 }

tests/ui/stability-attribute/stability-in-private-module.stderr

@@ -1,7 +1,7 @@
 error[E0603]: module `sys` is private
   --> $DIR/stability-in-private-module.rs:2:18
    |
-LL |     let _ = std::sys::os::errno();
+LL |     let _ = std::sys::io::errno();
    |                  ^^^      ----- function `errno` is not publicly re-exported
    |                  |
    |                  private module