XFAIL task-comm-5, it's burning the tinderboxes.

Author: graydon

src/Makefile

@@ -379,6 +379,7 @@ TEST_XFAILS_X86 :=  $(MUT_BOX_XFAILS) \
                     test/run-pass/vec-slice.rs \
                     test/run-pass/task-comm-2.rs \
                     test/run-pass/task-comm-3.rs \
+                    test/run-pass/task-comm-5.rs \
                     test/compile-fail/bad-recv.rs \
                     test/compile-fail/bad-send.rs \
                     test/compile-fail/infinite-tag-type-recursion.rs \