XFAIL task-comm-15

Fails consistently on windows

Author: brson

src/test/run-pass/task-comm-15.rs

@@ -1,3 +1,7 @@
+// xfail-stage1
+// xfail-stage2
+// xfail-stage3
+
 use std;
 import std::comm;
 import std::task;