Eliminate some extraneous curly brackets inside invocations of macro_rules!.

src/libcore/task.rs

@@ -1033,7 +1033,7 @@ fn kill_taskgroup(state: TaskGroupInner, me: *rust_task, is_main: bool) {
 // a proper closure because the #[test]s won't understand. Have to fake it.
 macro_rules! taskgroup_key (
     // Use a "code pointer" value that will never be a real code pointer.
-    {} => (unsafe::transmute((-2 as uint, 0u)))
+    () => (unsafe::transmute((-2 as uint, 0u)))
 )
 
 fn gen_child_taskgroup(linked: bool, supervised: bool)