Feature gate #[static_assert].

The API this exposes is a little strange (being attached to `static`s),
so it makes sense to conservatively feature gate it. If it is highly
popular, it is possible to reverse this gating.

src/test/run-pass/static-assert.rs

@@ -8,6 +8,8 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+#![feature(static_assert)]
+
 #[static_assert]
 static b: bool = true;