test: XFAIL test issue-4120 due to Valgrind complaints.

Author: pcwalton

src/test/run-pass/issue-4120.rs

@@ -1,3 +1,7 @@
+// xfail-test
+
+// XFAIL'd due to Valgrind complaining about TLS loss.
+
 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
 // file at the top-level directory of this distribution and at
 // http://rust-lang.org/COPYRIGHT.