xfail-fast the new test for windows compatibility

Author: SiegeLord

src/test/run-pass/struct_variant_xc.rs

@@ -8,6 +8,7 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+// xfail-fast - check-fast doesn't understand aux-build
 // aux-build:struct_variant_xc_aux.rs
 extern mod struct_variant_xc_aux;