Add missing test for lang item feature gate

We recently modified all tests with lang items to add the lang feature
gate switch but we did not introduce any test to check that a proper
error message is printed when this switch is not used.

gcc/testsuite/ChangeLog:

	* rust/compile/lang_feature_gate.rs: New test.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>

Author: P-E-P

gcc/testsuite/rust/compile/lang_feature_gate.rs

@@ -0,0 +1,2 @@
+#[lang = "sized"] // { dg-error "lang items are subject to change. add .#!.feature.lang_items... to the crate attributes to enable" }
+pub trait Sized {}