test: Un-xfail alias-uninit-value.rs. Closes #374.

This started working correctly at some point.

Author: brson

src/test/run-pass/alias-uninit-value.rs

@@ -1,7 +1,3 @@
-// xfail-stage0
-// xfail-stage1
-// xfail-stage2
-
 // Regression test for issue #374
 
 use std;