Update dead-code-closure-bang

This stdlib function went away, so update it to use current syntax.

src/test/compile-fail/dead-code-closure-bang.rs

@@ -15,5 +15,5 @@
 fn main() {
     let x = || panic!();
     x();
-    std::io::println("Foo bar"); //~ ERROR: unreachable statement
+    println!("Foo bar"); //~ ERROR: unreachable statement
 }