annotate unsafe_fields test with #![feature(unsafe_fields)]

I want to make it clearer that these tests are associated with the
unstable `unsafe_fields` feature.

Author: ytmimi

tests/source/unsafe-field.rs

@@ -1,3 +1,5 @@
+#![feature(unsafe_fields)]
+
 struct Foo {
     unsafe
     field: (),

tests/target/unsafe-field.rs

@@ -1,3 +1,5 @@
+#![feature(unsafe_fields)]
+
 struct Foo {
     unsafe field: (),
 }