Add xfailed test for #3908

Author: brson

src/test/auxiliary/pub_use_xcrate1.rs

@@ -0,0 +1,3 @@
+pub struct Foo {
+    name: int
+}

src/test/auxiliary/pub_use_xcrate2.rs

@@ -0,0 +1,3 @@
+extern mod pub_use_xcrate1;
+
+pub use pub_use_xcrate1::Foo;

src/test/run-pass/pub-use-xcrate.rs

@@ -0,0 +1,14 @@
+// xfail-test Issue #3908
+// aux-build:pub_use_xcrate1.rs
+// aux-build:pub_use_xcrate2.rs
+
+extern mod pub_use_xcrate2;
+
+use pub_use_xcrate2::Foo;
+
+fn main() {
+    let foo: Foo = Foo {
+        name: 0
+    };
+}
+